From: Martin Schwidefsky <[email protected]>
For s390 there is one additional byte associated with each page,
the storage key. This byte contains the referenced and changed
bits and needs to be included into the hibernation image.
If the storage keys are not restored to their previous state all
original pages would appear to be dirty. This can cause
inconsistencies e.g. with read-only filesystems.
Cc: Pavel Machek <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Martin Schwidefsky <[email protected]>
---
arch/s390/kernel/swsusp_asm64.S | 3
kernel/power/snapshot.c | 148 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 151 insertions(+)
diff -urpN linux-2.6/arch/s390/kernel/swsusp_asm64.S linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S
--- linux-2.6/arch/s390/kernel/swsusp_asm64.S 2011-05-19 06:06:34.000000000 +0200
+++ linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S 2011-06-06 11:14:43.000000000 +0200
@@ -138,11 +138,14 @@ swsusp_arch_resume:
0:
lg %r2,8(%r1)
lg %r4,0(%r1)
+ iske %r0,%r4
lghi %r3,PAGE_SIZE
lghi %r5,PAGE_SIZE
1:
mvcle %r2,%r4,0
jo 1b
+ lg %r2,8(%r1)
+ sske %r0,%r2
lg %r1,16(%r1)
ltgr %r1,%r1
jnz 0b
diff -urpN linux-2.6/kernel/power/snapshot.c linux-2.6-patched/kernel/power/snapshot.c
--- linux-2.6/kernel/power/snapshot.c 2011-06-06 11:14:39.000000000 +0200
+++ linux-2.6-patched/kernel/power/snapshot.c 2011-06-06 11:14:43.000000000 +0200
@@ -1022,6 +1022,137 @@ static inline void copy_data_page(unsign
}
#endif /* CONFIG_HIGHMEM */
+#ifdef CONFIG_S390
+/*
+ * For s390 there is one additional byte associated with each page,
+ * the storage key. The key consists of the access-control bits
+ * (alias the key), the fetch-protection bit, the referenced bit
+ * and the change bit (alias dirty bit). Linux uses only the
+ * referenced bit and the change bit for pages in the page cache.
+ * Any modification of a page will set the change bit, any access
+ * sets the referenced bit. Overindication of the referenced bits
+ * after a hibernation cycle does not cause any harm but the
+ * overindication of the change bits would cause trouble.
+ * Therefore it is necessary to include the storage keys in the
+ * hibernation image. The storage keys are saved to the most
+ * significant byte of each page frame number in the list of pfns
+ * in the hibernation image.
+ */
+
+/*
+ * Key storage is allocated as a linked list of pages.
+ * The size of the keys array is (PAGE_SIZE - sizeof(long))
+ */
+struct page_key_data {
+ struct page_key_data *next;
+ unsigned char data[];
+};
+
+#define PAGE_KEY_DATA_SIZE (PAGE_SIZE - sizeof(struct page_key_data *))
+
+static struct page_key_data *page_key_data;
+static struct page_key_data *page_key_rp, *page_key_wp;
+static unsigned long page_key_rx, page_key_wx;
+
+/*
+ * For each page in the hibernation image one additional byte is
+ * stored in the most significant byte of the page frame number.
+ * On suspend no additional memory is required but on resume the
+ * keys need to be memorized until the page data has been restored.
+ * Only then can the storage keys be set to their old state.
+ */
+static inline unsigned long page_key_additional_pages(unsigned long pages)
+{
+ return DIV_ROUND_UP(pages, PAGE_KEY_DATA_SIZE);
+}
+
+/*
+ * Free page_key_data list of arrays.
+ */
+static void page_key_free(void)
+{
+ struct page_key_data *pkd;
+
+ while (page_key_data) {
+ pkd = page_key_data;
+ page_key_data = pkd->next;
+ free_page((unsigned long) pkd);
+ }
+}
+
+/*
+ * Allocate page_key_data list of arrays with enough room to store
+ * one byte for each page in the hibernation image.
+ */
+static int page_key_alloc(unsigned long pages)
+{
+ struct page_key_data *pk;
+ unsigned long size;
+
+ size = DIV_ROUND_UP(pages, PAGE_KEY_DATA_SIZE);
+ while (size--) {
+ pk = (struct page_key_data *) get_zeroed_page(GFP_KERNEL);
+ if (!pk) {
+ page_key_free();
+ return -ENOMEM;
+ }
+ pk->next = page_key_data;
+ page_key_data = pk;
+ }
+ page_key_rp = page_key_wp = page_key_data;
+ page_key_rx = page_key_wx = 0;
+ return 0;
+}
+
+/*
+ * Save the storage key into the upper 8 bits of the page frame number.
+ */
+static inline void page_key_read(unsigned long *pfn)
+{
+ unsigned long addr;
+
+ addr = (unsigned long) page_address(pfn_to_page(*pfn));
+ *(unsigned char *) pfn = (unsigned char) page_get_storage_key(addr);
+}
+
+/*
+ * Extract the storage key from the upper 8 bits of the page frame number
+ * and store it in the page_key_data list of arrays.
+ */
+static inline void page_key_memorize(unsigned long *pfn)
+{
+ page_key_wp->data[page_key_wx] = *(unsigned char *) pfn;
+ *(unsigned char *) pfn = 0;
+ if (++page_key_wx < PAGE_KEY_DATA_SIZE)
+ return;
+ page_key_wp = page_key_wp->next;
+ page_key_wx = 0;
+}
+
+/*
+ * Get the next key from the page_key_data list of arrays and set the
+ * storage key of the page referred by @address. If @address refers to
+ * a "safe" page the swsusp_arch_resume code will transfer the storage
+ * key from the buffer page to the original page.
+ */
+static void page_key_write(void *address)
+{
+ page_set_storage_key((unsigned long) address,
+ page_key_rp->data[page_key_rx], 0);
+ if (++page_key_rx >= PAGE_KEY_DATA_SIZE)
+ return;
+ page_key_rp = page_key_rp->next;
+ page_key_rx = 0;
+}
+#else
+static unsigned long page_key_additional_pages(unsigned long pages) { return 0; }
+static inline int page_key_alloc(unsigned long pages) { return 0; }
+static inline void page_key_free(void) { }
+static inline void page_key_read(unsigned long *pfn) { }
+static inline void page_key_memorize(unsigned long *pfn) { }
+static inline void page_key_write(void *address) { }
+#endif
+
static void
copy_data_pages(struct memory_bitmap *copy_bm, struct memory_bitmap *orig_bm)
{
@@ -1335,6 +1466,9 @@ int hibernate_preallocate_memory(void)
count += highmem;
count -= totalreserve_pages;
+ /* Add number of pages required for storage keys (s390 only). */
+ size += page_key_additional_pages(saveable);
+
/* Compute the maximum number of saveable pages to leave in memory. */
max_size = (count - (size + PAGES_FOR_IO)) / 2
- 2 * DIV_ROUND_UP(reserved_size, PAGE_SIZE);
@@ -1658,6 +1792,8 @@ pack_pfns(unsigned long *buf, struct mem
buf[j] = memory_bm_next_pfn(bm);
if (unlikely(buf[j] == BM_END_OF_MAP))
break;
+ /* Save storage key for data page (s390 only). */
+ page_key_read(buf + j);
}
}
@@ -1817,6 +1953,9 @@ static int unpack_orig_pfns(unsigned lon
if (unlikely(buf[j] == BM_END_OF_MAP))
break;
+ /* Extract and buffer storage key for data page (s390 only). */
+ page_key_memorize(buf + j);
+
if (memory_bm_pfn_present(bm, buf[j]))
memory_bm_set_bit(bm, buf[j]);
else
@@ -2219,6 +2358,10 @@ int snapshot_write_next(struct snapshot_
if (error)
return error;
+ error = page_key_alloc(nr_copy_pages);
+ if (error)
+ return error;
+
} else if (handle->cur <= nr_meta_pages + 1) {
error = unpack_orig_pfns(buffer, ©_bm);
if (error)
@@ -2239,6 +2382,8 @@ int snapshot_write_next(struct snapshot_
}
} else {
copy_last_highmem_page();
+ /* Restore storage key for data page (s390 only). */
+ page_key_write(handle->buffer);
handle->buffer = get_buffer(&orig_bm, &ca);
if (IS_ERR(handle->buffer))
return PTR_ERR(handle->buffer);
@@ -2260,6 +2405,9 @@ int snapshot_write_next(struct snapshot_
void snapshot_write_finalize(struct snapshot_handle *handle)
{
copy_last_highmem_page();
+ /* Restore storage key for data page (s390 only). */
+ page_key_write(handle->buffer);
+ page_key_free();
/* Free only if we have loaded the image entirely */
if (handle->cur > 1 && handle->cur > nr_meta_pages + nr_copy_pages) {
memory_bm_free(&orig_bm, PG_UNSAFE_CLEAR);
On Wednesday, June 08, 2011, Martin Schwidefsky wrote:
> From: Martin Schwidefsky <[email protected]>
>
> For s390 there is one additional byte associated with each page,
> the storage key. This byte contains the referenced and changed
> bits and needs to be included into the hibernation image.
> If the storage keys are not restored to their previous state all
> original pages would appear to be dirty. This can cause
> inconsistencies e.g. with read-only filesystems.
>
> Cc: Pavel Machek <[email protected]>
> Cc: Rafael J. Wysocki <[email protected]>
> Cc: Jiri Slaby <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Martin Schwidefsky <[email protected]>
> ---
>
> arch/s390/kernel/swsusp_asm64.S | 3
> kernel/power/snapshot.c | 148 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 151 insertions(+)
>
> diff -urpN linux-2.6/arch/s390/kernel/swsusp_asm64.S linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S
> --- linux-2.6/arch/s390/kernel/swsusp_asm64.S 2011-05-19 06:06:34.000000000 +0200
> +++ linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S 2011-06-06 11:14:43.000000000 +0200
> @@ -138,11 +138,14 @@ swsusp_arch_resume:
> 0:
> lg %r2,8(%r1)
> lg %r4,0(%r1)
> + iske %r0,%r4
> lghi %r3,PAGE_SIZE
> lghi %r5,PAGE_SIZE
> 1:
> mvcle %r2,%r4,0
> jo 1b
> + lg %r2,8(%r1)
> + sske %r0,%r2
> lg %r1,16(%r1)
> ltgr %r1,%r1
> jnz 0b
I cannot comment on the assembly.
> diff -urpN linux-2.6/kernel/power/snapshot.c linux-2.6-patched/kernel/power/snapshot.c
> --- linux-2.6/kernel/power/snapshot.c 2011-06-06 11:14:39.000000000 +0200
> +++ linux-2.6-patched/kernel/power/snapshot.c 2011-06-06 11:14:43.000000000 +0200
> @@ -1022,6 +1022,137 @@ static inline void copy_data_page(unsign
> }
> #endif /* CONFIG_HIGHMEM */
>
> +#ifdef CONFIG_S390
> +/*
> + * For s390 there is one additional byte associated with each page,
> + * the storage key. The key consists of the access-control bits
> + * (alias the key), the fetch-protection bit, the referenced bit
> + * and the change bit (alias dirty bit). Linux uses only the
> + * referenced bit and the change bit for pages in the page cache.
> + * Any modification of a page will set the change bit, any access
> + * sets the referenced bit. Overindication of the referenced bits
> + * after a hibernation cycle does not cause any harm but the
> + * overindication of the change bits would cause trouble.
> + * Therefore it is necessary to include the storage keys in the
> + * hibernation image. The storage keys are saved to the most
> + * significant byte of each page frame number in the list of pfns
> + * in the hibernation image.
> + */
Let me say that is not exactly straightforward. :-)
One thing I don't really understand is where those storage keys are normally
stored so that they aren't present in the image without this additional
mechanism. Could you explain that a bit more, please?
> +
> +/*
> + * Key storage is allocated as a linked list of pages.
> + * The size of the keys array is (PAGE_SIZE - sizeof(long))
> + */
> +struct page_key_data {
> + struct page_key_data *next;
> + unsigned char data[];
> +};
This seems to be similar to the data structure for the saving of ACPI NVS
memory, so perhaps it's possible to combine the two.
> +#define PAGE_KEY_DATA_SIZE (PAGE_SIZE - sizeof(struct page_key_data *))
> +
> +static struct page_key_data *page_key_data;
> +static struct page_key_data *page_key_rp, *page_key_wp;
> +static unsigned long page_key_rx, page_key_wx;
> +
> +/*
> + * For each page in the hibernation image one additional byte is
> + * stored in the most significant byte of the page frame number.
I'm not sure it's really worth the complication. If you simply store those
keys in separete pages, you'd need one additional page per PAGE_SIZE
page frames, so given PAGE_SIZE = 4096 that's one page per 16 MB of memory,
which is not a whole lot (64 extra pages per GB if I'm not mistaken).
It seems that doing it will simplify things quite a git.
Apart from this, I'm not sure that kernel/power/snapshot.c is the right
place for all this S390-specific code. I'd rather see it in arch/s390.
Thanks,
Rafael
Hi Rafael,
first of all: thanks for the review.
On Sun, 12 Jun 2011 14:41:34 +0200
"Rafael J. Wysocki" <[email protected]> wrote:
> > diff -urpN linux-2.6/arch/s390/kernel/swsusp_asm64.S linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S
> > --- linux-2.6/arch/s390/kernel/swsusp_asm64.S 2011-05-19 06:06:34.000000000 +0200
> > +++ linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S 2011-06-06 11:14:43.000000000 +0200
> > @@ -138,11 +138,14 @@ swsusp_arch_resume:
> > 0:
> > lg %r2,8(%r1)
> > lg %r4,0(%r1)
> > + iske %r0,%r4
> > lghi %r3,PAGE_SIZE
> > lghi %r5,PAGE_SIZE
> > 1:
> > mvcle %r2,%r4,0
> > jo 1b
> > + lg %r2,8(%r1)
> > + sske %r0,%r2
> > lg %r1,16(%r1)
> > ltgr %r1,%r1
> > jnz 0b
>
> I cannot comment on the assembly.
The swsusp_arch_resume code copies the safe pages to their final target
page. Writing to a page sets the dirty bit in the storage key. Which
means that the storage key needs to be restored >after< the copy has
been done. To avoid having to deal with additional pages where the
storage key are kept while the copy is still pending I simply set the
storage key of the safe page and transfer it from the safe page to the
final target page after the copy has been done. That is what the iske
and the sske instruction are doing.
> > diff -urpN linux-2.6/kernel/power/snapshot.c linux-2.6-patched/kernel/power/snapshot.c
> > --- linux-2.6/kernel/power/snapshot.c 2011-06-06 11:14:39.000000000 +0200
> > +++ linux-2.6-patched/kernel/power/snapshot.c 2011-06-06 11:14:43.000000000 +0200
> > @@ -1022,6 +1022,137 @@ static inline void copy_data_page(unsign
> > }
> > #endif /* CONFIG_HIGHMEM */
> >
> > +#ifdef CONFIG_S390
> > +/*
> > + * For s390 there is one additional byte associated with each page,
> > + * the storage key. The key consists of the access-control bits
> > + * (alias the key), the fetch-protection bit, the referenced bit
> > + * and the change bit (alias dirty bit). Linux uses only the
> > + * referenced bit and the change bit for pages in the page cache.
> > + * Any modification of a page will set the change bit, any access
> > + * sets the referenced bit. Overindication of the referenced bits
> > + * after a hibernation cycle does not cause any harm but the
> > + * overindication of the change bits would cause trouble.
> > + * Therefore it is necessary to include the storage keys in the
> > + * hibernation image. The storage keys are saved to the most
> > + * significant byte of each page frame number in the list of pfns
> > + * in the hibernation image.
> > + */
>
> Let me say that is not exactly straightforward. :-)
>
> One thing I don't really understand is where those storage keys are normally
> stored so that they aren't present in the image without this additional
> mechanism. Could you explain that a bit more, please?
The storage keys are outside of the directly addressable memory, the only
way to get to them is with special instructions:
iske - insert storage key extended, ivsk - insert virtual storage key,
sske - set storage key extended, and rrbe - reset reference bit extended.
The storage key of a page has 7 bits, 4 bit for access-control, the
fetch-protection-bit, the reference-bit and the change-bit. In Linux only
the reference and change bits are used.
These bits are per physical page, one of the major differences of the s390
machines compared to other architectures. We had a number of interesting
problems because of pte dirty & referenced vs page dirty & referenced.
For e.g. x86 a page is implicitly clean if it has no mapper. On s390 is
depends on the contents of the storage key. Which is why SetPageUptodate
clears the storage key.
In regard to hibernation the important aspect is that each and every write
to a page sets the dirty bit even if it is done by I/O. The restore of the
hibernation image therefore makes all pages dirty. To get the correct
original combination of page content and storage key content the storage
key needs to be set after the last access to the page content.
> > +
> > +/*
> > + * Key storage is allocated as a linked list of pages.
> > + * The size of the keys array is (PAGE_SIZE - sizeof(long))
> > + */
> > +struct page_key_data {
> > + struct page_key_data *next;
> > + unsigned char data[];
> > +};
>
> This seems to be similar to the data structure for the saving of ACPI NVS
> memory, so perhaps it's possible to combine the two.
Almost the same, I thought about ways to merge them. I dropped the idea in
order to keep the patch as small as possible.
> > +#define PAGE_KEY_DATA_SIZE (PAGE_SIZE - sizeof(struct page_key_data *))
> > +
> > +static struct page_key_data *page_key_data;
> > +static struct page_key_data *page_key_rp, *page_key_wp;
> > +static unsigned long page_key_rx, page_key_wx;
> > +
> > +/*
> > + * For each page in the hibernation image one additional byte is
> > + * stored in the most significant byte of the page frame number.
>
> I'm not sure it's really worth the complication. If you simply store those
> keys in separete pages, you'd need one additional page per PAGE_SIZE
> page frames, so given PAGE_SIZE = 4096 that's one page per 16 MB of memory,
> which is not a whole lot (64 extra pages per GB if I'm not mistaken).
>
> It seems that doing it will simplify things quite a git.
One of the versions of the patch actually did that and it turned out to
be much larger then the proposed solution. That older version had another
step in snapshot_read_next / snapshot_write_next to copy the storage keys
to pages allocated for that purpose. I had some trouble with the way how
the code deals with the orig_bm and the copy_bm though. Not trivial code..
In the end I decided to use some of the unused 12 bits in the pfn list
entries, it avoids any structural changes to the snapshot code and it
resulted in the smallest patch (at least so far). One thing that seemed
important to me was to make the patch obviously correct for the other
architectures, for !s390 the new functions are nops and there is no
structural change.
> Apart from this, I'm not sure that kernel/power/snapshot.c is the right
> place for all this S390-specific code. I'd rather see it in arch/s390.
The alternative would be to move the s390 specific page_key_xxx functions
to some architecture file. The hooks in snapshot.c would have to stay
though. Unfortunately they are in odd places and you can only understand
them in the context of the s390 storage keys.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
On Tuesday, June 14, 2011, Martin Schwidefsky wrote:
> Hi Rafael,
Hi,
> first of all: thanks for the review.
>
> On Sun, 12 Jun 2011 14:41:34 +0200
> "Rafael J. Wysocki" <[email protected]> wrote:
>
> > > diff -urpN linux-2.6/arch/s390/kernel/swsusp_asm64.S linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S
> > > --- linux-2.6/arch/s390/kernel/swsusp_asm64.S 2011-05-19 06:06:34.000000000 +0200
> > > +++ linux-2.6-patched/arch/s390/kernel/swsusp_asm64.S 2011-06-06 11:14:43.000000000 +0200
> > > @@ -138,11 +138,14 @@ swsusp_arch_resume:
> > > 0:
> > > lg %r2,8(%r1)
> > > lg %r4,0(%r1)
> > > + iske %r0,%r4
> > > lghi %r3,PAGE_SIZE
> > > lghi %r5,PAGE_SIZE
> > > 1:
> > > mvcle %r2,%r4,0
> > > jo 1b
> > > + lg %r2,8(%r1)
> > > + sske %r0,%r2
> > > lg %r1,16(%r1)
> > > ltgr %r1,%r1
> > > jnz 0b
> >
> > I cannot comment on the assembly.
>
> The swsusp_arch_resume code copies the safe pages to their final target
> page. Writing to a page sets the dirty bit in the storage key. Which
> means that the storage key needs to be restored >after< the copy has
> been done. To avoid having to deal with additional pages where the
> storage key are kept while the copy is still pending I simply set the
> storage key of the safe page and transfer it from the safe page to the
> final target page after the copy has been done. That is what the iske
> and the sske instruction are doing.
I see.
> > > diff -urpN linux-2.6/kernel/power/snapshot.c linux-2.6-patched/kernel/power/snapshot.c
> > > --- linux-2.6/kernel/power/snapshot.c 2011-06-06 11:14:39.000000000 +0200
> > > +++ linux-2.6-patched/kernel/power/snapshot.c 2011-06-06 11:14:43.000000000 +0200
> > > @@ -1022,6 +1022,137 @@ static inline void copy_data_page(unsign
> > > }
> > > #endif /* CONFIG_HIGHMEM */
> > >
> > > +#ifdef CONFIG_S390
> > > +/*
> > > + * For s390 there is one additional byte associated with each page,
> > > + * the storage key. The key consists of the access-control bits
> > > + * (alias the key), the fetch-protection bit, the referenced bit
> > > + * and the change bit (alias dirty bit). Linux uses only the
> > > + * referenced bit and the change bit for pages in the page cache.
> > > + * Any modification of a page will set the change bit, any access
> > > + * sets the referenced bit. Overindication of the referenced bits
> > > + * after a hibernation cycle does not cause any harm but the
> > > + * overindication of the change bits would cause trouble.
> > > + * Therefore it is necessary to include the storage keys in the
> > > + * hibernation image. The storage keys are saved to the most
> > > + * significant byte of each page frame number in the list of pfns
> > > + * in the hibernation image.
> > > + */
> >
> > Let me say that is not exactly straightforward. :-)
> >
> > One thing I don't really understand is where those storage keys are normally
> > stored so that they aren't present in the image without this additional
> > mechanism. Could you explain that a bit more, please?
>
> The storage keys are outside of the directly addressable memory, the only
> way to get to them is with special instructions:
> iske - insert storage key extended, ivsk - insert virtual storage key,
> sske - set storage key extended, and rrbe - reset reference bit extended.
> The storage key of a page has 7 bits, 4 bit for access-control, the
> fetch-protection-bit, the reference-bit and the change-bit. In Linux only
> the reference and change bits are used.
OK, so it looks like we would only need to store 2 bits per page in additional
allocations.
> These bits are per physical page, one of the major differences of the s390
> machines compared to other architectures. We had a number of interesting
> problems because of pte dirty & referenced vs page dirty & referenced.
> For e.g. x86 a page is implicitly clean if it has no mapper. On s390 is
> depends on the contents of the storage key. Which is why SetPageUptodate
> clears the storage key.
>
> In regard to hibernation the important aspect is that each and every write
> to a page sets the dirty bit even if it is done by I/O. The restore of the
> hibernation image therefore makes all pages dirty. To get the correct
> original combination of page content and storage key content the storage
> key needs to be set after the last access to the page content.
What, in your opinion, is the right time for restoring the original storage
keys after the image has been loaded? It seems that should happen after
passing control to the hibernated kernel.
> > > +
> > > +/*
> > > + * Key storage is allocated as a linked list of pages.
> > > + * The size of the keys array is (PAGE_SIZE - sizeof(long))
> > > + */
> > > +struct page_key_data {
> > > + struct page_key_data *next;
> > > + unsigned char data[];
> > > +};
> >
> > This seems to be similar to the data structure for the saving of ACPI NVS
> > memory, so perhaps it's possible to combine the two.
>
> Almost the same, I thought about ways to merge them. I dropped the idea in
> order to keep the patch as small as possible.
I think, however, that we really should try to merge them. The only
difference seems to be how the additionally allocated pages will be populated
and what's going to happen to their contents during restore.
ACPI will simply copy the NVS memory to those pages, while S390 will save
the relevant storage key bits in there.
> > > +#define PAGE_KEY_DATA_SIZE (PAGE_SIZE - sizeof(struct page_key_data *))
> > > +
> > > +static struct page_key_data *page_key_data;
> > > +static struct page_key_data *page_key_rp, *page_key_wp;
> > > +static unsigned long page_key_rx, page_key_wx;
> > > +
> > > +/*
> > > + * For each page in the hibernation image one additional byte is
> > > + * stored in the most significant byte of the page frame number.
> >
> > I'm not sure it's really worth the complication. If you simply store those
> > keys in separete pages, you'd need one additional page per PAGE_SIZE
> > page frames, so given PAGE_SIZE = 4096 that's one page per 16 MB of memory,
> > which is not a whole lot (64 extra pages per GB if I'm not mistaken).
> >
> > It seems that doing it will simplify things quite a git.
>
> One of the versions of the patch actually did that and it turned out to
> be much larger then the proposed solution. That older version had another
> step in snapshot_read_next / snapshot_write_next to copy the storage keys
> to pages allocated for that purpose. I had some trouble with the way how
> the code deals with the orig_bm and the copy_bm though. Not trivial code..
Well, I'm sure we can handle that. :-)
I thought of the following approach:
* Allocate memory for saving the relevant bits of the storage keys before
the memory preallocation kicks in.
* Save the storage key bits to that memory before creating the image (be
careful about the storage keys of the pages they are being stored into).
* After getting control from the boot kernel, restore the storage key bits
saved before creating the image.
* Free additional memory used for storing them.
This way we wouldn't have to worry about any interactions with
snapshot_read_next / snapshot_write_next etc.
Thanks,
Rafael
On Tue, 14 Jun 2011 22:50:14 +0200
"Rafael J. Wysocki" <[email protected]> wrote:
> On Tuesday, June 14, 2011, Martin Schwidefsky wrote:
> > Hi Rafael,
>
> > On Sun, 12 Jun 2011 14:41:34 +0200
> > "Rafael J. Wysocki" <[email protected]> wrote:
> >
> > > > diff -urpN linux-2.6/kernel/power/snapshot.c linux-2.6-patched/kernel/power/snapshot.c
> > > > --- linux-2.6/kernel/power/snapshot.c 2011-06-06 11:14:39.000000000 +0200
> > > > +++ linux-2.6-patched/kernel/power/snapshot.c 2011-06-06 11:14:43.000000000 +0200
> > > > @@ -1022,6 +1022,137 @@ static inline void copy_data_page(unsign
> > > > }
> > > > #endif /* CONFIG_HIGHMEM */
> > > >
> > > > +#ifdef CONFIG_S390
> > > > +/*
> > > > + * For s390 there is one additional byte associated with each page,
> > > > + * the storage key. The key consists of the access-control bits
> > > > + * (alias the key), the fetch-protection bit, the referenced bit
> > > > + * and the change bit (alias dirty bit). Linux uses only the
> > > > + * referenced bit and the change bit for pages in the page cache.
> > > > + * Any modification of a page will set the change bit, any access
> > > > + * sets the referenced bit. Overindication of the referenced bits
> > > > + * after a hibernation cycle does not cause any harm but the
> > > > + * overindication of the change bits would cause trouble.
> > > > + * Therefore it is necessary to include the storage keys in the
> > > > + * hibernation image. The storage keys are saved to the most
> > > > + * significant byte of each page frame number in the list of pfns
> > > > + * in the hibernation image.
> > > > + */
> > >
> > > Let me say that is not exactly straightforward. :-)
> > >
> > > One thing I don't really understand is where those storage keys are normally
> > > stored so that they aren't present in the image without this additional
> > > mechanism. Could you explain that a bit more, please?
> >
> > The storage keys are outside of the directly addressable memory, the only
> > way to get to them is with special instructions:
> > iske - insert storage key extended, ivsk - insert virtual storage key,
> > sske - set storage key extended, and rrbe - reset reference bit extended.
> > The storage key of a page has 7 bits, 4 bit for access-control, the
> > fetch-protection-bit, the reference-bit and the change-bit. In Linux only
> > the reference and change bits are used.
>
> OK, so it looks like we would only need to store 2 bits per page in additional
> allocations.
For now we'd only need two bits, but with KVM guests we will have to store the
complete storage key. The guest OS could make use of all the bits in the storage
key, the KVM host needs to preserve them.
> > These bits are per physical page, one of the major differences of the s390
> > machines compared to other architectures. We had a number of interesting
> > problems because of pte dirty & referenced vs page dirty & referenced.
> > For e.g. x86 a page is implicitly clean if it has no mapper. On s390 is
> > depends on the contents of the storage key. Which is why SetPageUptodate
> > clears the storage key.
> >
> > In regard to hibernation the important aspect is that each and every write
> > to a page sets the dirty bit even if it is done by I/O. The restore of the
> > hibernation image therefore makes all pages dirty. To get the correct
> > original combination of page content and storage key content the storage
> > key needs to be set after the last access to the page content.
>
> What, in your opinion, is the right time for restoring the original storage
> keys after the image has been loaded? It seems that should happen after
> passing control to the hibernated kernel.
Anytime after the final write of the page content completed and before the
hibernated kernel gets control (or shortly after in the resume code of the
restored kernel).
> > > > +
> > > > +/*
> > > > + * Key storage is allocated as a linked list of pages.
> > > > + * The size of the keys array is (PAGE_SIZE - sizeof(long))
> > > > + */
> > > > +struct page_key_data {
> > > > + struct page_key_data *next;
> > > > + unsigned char data[];
> > > > +};
> > >
> > > This seems to be similar to the data structure for the saving of ACPI NVS
> > > memory, so perhaps it's possible to combine the two.
> >
> > Almost the same, I thought about ways to merge them. I dropped the idea in
> > order to keep the patch as small as possible.
>
> I think, however, that we really should try to merge them. The only
> difference seems to be how the additionally allocated pages will be populated
> and what's going to happen to their contents during restore.
>
> ACPI will simply copy the NVS memory to those pages, while S390 will save
> the relevant storage key bits in there.
One complication to keep in mind is that we need to know which storage key
goes to which page frame. We need something like the orig_bm/copy_bm or
we'd have to store the pfn with the key. Simply storing the key for every
page will make the array unnecessarily big.
> > > > +#define PAGE_KEY_DATA_SIZE (PAGE_SIZE - sizeof(struct page_key_data *))
> > > > +
> > > > +static struct page_key_data *page_key_data;
> > > > +static struct page_key_data *page_key_rp, *page_key_wp;
> > > > +static unsigned long page_key_rx, page_key_wx;
> > > > +
> > > > +/*
> > > > + * For each page in the hibernation image one additional byte is
> > > > + * stored in the most significant byte of the page frame number.
> > >
> > > I'm not sure it's really worth the complication. If you simply store those
> > > keys in separete pages, you'd need one additional page per PAGE_SIZE
> > > page frames, so given PAGE_SIZE = 4096 that's one page per 16 MB of memory,
> > > which is not a whole lot (64 extra pages per GB if I'm not mistaken).
> > >
> > > It seems that doing it will simplify things quite a git.
> >
> > One of the versions of the patch actually did that and it turned out to
> > be much larger then the proposed solution. That older version had another
> > step in snapshot_read_next / snapshot_write_next to copy the storage keys
> > to pages allocated for that purpose. I had some trouble with the way how
> > the code deals with the orig_bm and the copy_bm though. Not trivial code..
>
> Well, I'm sure we can handle that. :-)
>
> I thought of the following approach:
>
> * Allocate memory for saving the relevant bits of the storage keys before
> the memory preallocation kicks in.
>
> * Save the storage key bits to that memory before creating the image (be
> careful about the storage keys of the pages they are being stored into).
>
> * After getting control from the boot kernel, restore the storage key bits
> saved before creating the image.
>
> * Free additional memory used for storing them.
>
> This way we wouldn't have to worry about any interactions with
> snapshot_read_next / snapshot_write_next etc.
Well before the preallocation kicked in we don't know which are the relevant
storage keys. The only other option would be to store all of them which I
consider to be a inferior solution.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
On Wednesday, June 15, 2011, Martin Schwidefsky wrote:
> On Tue, 14 Jun 2011 22:50:14 +0200
...
> > Well, I'm sure we can handle that. :-)
> >
> > I thought of the following approach:
> >
> > * Allocate memory for saving the relevant bits of the storage keys before
> > the memory preallocation kicks in.
> >
> > * Save the storage key bits to that memory before creating the image (be
> > careful about the storage keys of the pages they are being stored into).
> >
> > * After getting control from the boot kernel, restore the storage key bits
> > saved before creating the image.
> >
> > * Free additional memory used for storing them.
> >
> > This way we wouldn't have to worry about any interactions with
> > snapshot_read_next / snapshot_write_next etc.
>
> Well before the preallocation kicked in we don't know which are the relevant
> storage keys. The only other option would be to store all of them which I
> consider to be a inferior solution.
OK, I see your point. You think it's necessary to save the entire storage key
for each page and you'd like to save storage keys only for those pages, whose
contents is going to be saved in the image.
Now, I need to look at your original patch again. :-)