LTP proc01 testcase has been observed to rarely trigger crashes
on arm64:
page_mapped+0x78/0xb4
stable_page_flags+0x27c/0x338
kpageflags_read+0xfc/0x164
proc_reg_read+0x7c/0xb8
__vfs_read+0x58/0x178
vfs_read+0x90/0x14c
SyS_read+0x60/0xc0
Issue is that page_mapped() assumes that if compound page is not
huge, then it must be THP. But if this is 'normal' compound page
(COMPOUND_PAGE_DTOR), then following loop can keep running until
it tries to read from memory that isn't mapped and triggers a panic:
for (i = 0; i < hpage_nr_pages(page); i++) {
if (atomic_read(&page[i]._mapcount) >= 0)
return true;
}
I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
with a custom kernel module [1] which:
- allocates compound page (PAGEC) of order 1
- allocates 2 normal pages (COPY), which are initialized to 0xff
(to satisfy _mapcount >= 0)
- 2 PAGEC page structs are copied to address of first COPY page
- second page of COPY is marked as not present
- call to page_mapped(COPY) now triggers fault on access to 2nd COPY
page at offset 0x30 (_mapcount)
[1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
This patch modifies page_mapped() to check for 'normal'
compound pages (COMPOUND_PAGE_DTOR).
Debugged-by: Laszlo Ersek <[email protected]>
Signed-off-by: Jan Stancek <[email protected]>
---
include/linux/mm.h | 9 +++++++++
mm/util.c | 2 ++
2 files changed, 11 insertions(+)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5411de93a363..18b0bb953f92 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -700,6 +700,15 @@ static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
return compound_page_dtors[page[1].compound_dtor];
}
+static inline int PageNormalCompound(struct page *page)
+{
+ if (!PageCompound(page))
+ return 0;
+
+ page = compound_head(page);
+ return page[1].compound_dtor == COMPOUND_PAGE_DTOR;
+}
+
static inline unsigned int compound_order(struct page *page)
{
if (!PageHead(page))
diff --git a/mm/util.c b/mm/util.c
index 8bf08b5b5760..06c1640cb7b3 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -478,6 +478,8 @@ bool page_mapped(struct page *page)
return true;
if (PageHuge(page))
return false;
+ if (PageNormalCompound(page))
+ return false;
for (i = 0; i < hpage_nr_pages(page); i++) {
if (atomic_read(&page[i]._mapcount) >= 0)
return true;
--
1.8.3.1
On Thu, Nov 29, 2018 at 10:53:48PM +0100, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running until
> it tries to read from memory that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd COPY
> page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> This patch modifies page_mapped() to check for 'normal'
> compound pages (COMPOUND_PAGE_DTOR).
>
> Debugged-by: Laszlo Ersek <[email protected]>
> Signed-off-by: Jan Stancek <[email protected]>
> ---
> include/linux/mm.h | 9 +++++++++
> mm/util.c | 2 ++
> 2 files changed, 11 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5411de93a363..18b0bb953f92 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -700,6 +700,15 @@ static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
> return compound_page_dtors[page[1].compound_dtor];
> }
>
> +static inline int PageNormalCompound(struct page *page)
> +{
> + if (!PageCompound(page))
> + return 0;
> +
> + page = compound_head(page);
> + return page[1].compound_dtor == COMPOUND_PAGE_DTOR;
> +}
> +
> static inline unsigned int compound_order(struct page *page)
> {
> if (!PageHead(page))
> diff --git a/mm/util.c b/mm/util.c
> index 8bf08b5b5760..06c1640cb7b3 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -478,6 +478,8 @@ bool page_mapped(struct page *page)
> return true;
> if (PageHuge(page))
> return false;
> + if (PageNormalCompound(page))
> + return false;
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
Thanks for catching this.
But I think the right fix would be to change the loop condition:
for (i = 0; i < (1 << compund_order(page)); i++) {
Non-THP compound page also can be mapped and we need to check mapcount of
subpages.
Any objections?
If not, please update the patch.
--
Kirill A. Shutemov
LTP proc01 testcase has been observed to rarely trigger crashes
on arm64:
page_mapped+0x78/0xb4
stable_page_flags+0x27c/0x338
kpageflags_read+0xfc/0x164
proc_reg_read+0x7c/0xb8
__vfs_read+0x58/0x178
vfs_read+0x90/0x14c
SyS_read+0x60/0xc0
Issue is that page_mapped() assumes that if compound page is not
huge, then it must be THP. But if this is 'normal' compound page
(COMPOUND_PAGE_DTOR), then following loop can keep running
(for HPAGE_PMD_NR iterations) until it tries to read from memory
that isn't mapped and triggers a panic:
for (i = 0; i < hpage_nr_pages(page); i++) {
if (atomic_read(&page[i]._mapcount) >= 0)
return true;
}
I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
with a custom kernel module [1] which:
- allocates compound page (PAGEC) of order 1
- allocates 2 normal pages (COPY), which are initialized to 0xff
(to satisfy _mapcount >= 0)
- 2 PAGEC page structs are copied to address of first COPY page
- second page of COPY is marked as not present
- call to page_mapped(COPY) now triggers fault on access to 2nd
COPY page at offset 0x30 (_mapcount)
[1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
Fix the loop to iterate for "1 << compound_order" pages.
Debugged-by: Laszlo Ersek <[email protected]>
Suggested-by: "Kirill A. Shutemov" <[email protected]>
Signed-off-by: Jan Stancek <[email protected]>
---
mm/util.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Changes in v2:
- change the loop instead so we check also mapcount of subpages
diff --git a/mm/util.c b/mm/util.c
index 8bf08b5b5760..5c9c7359ee8a 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
return true;
if (PageHuge(page))
return false;
- for (i = 0; i < hpage_nr_pages(page); i++) {
+ for (i = 0; i < (1 << compound_order(page)); i++) {
if (atomic_read(&page[i]._mapcount) >= 0)
return true;
}
--
1.8.3.1
On Fri 30-11-18 13:06:57, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running
> (for HPAGE_PMD_NR iterations) until it tries to read from memory
> that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd
> COPY page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> Fix the loop to iterate for "1 << compound_order" pages.
This is much less magic than the previous version. It is still not clear
to me how is mapping higher order pages to page tables other than THP
though. So a more detailed information about the source would bre really
welcome. Once we know that we can add a Fixes tag and also mark the
patch for stable because that sounds like a stable material.
> Debugged-by: Laszlo Ersek <[email protected]>
> Suggested-by: "Kirill A. Shutemov" <[email protected]>
> Signed-off-by: Jan Stancek <[email protected]>
The patch looks sensible to me
Acked-by: Michal Hocko <[email protected]>
Thanks!
> ---
> mm/util.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Changes in v2:
> - change the loop instead so we check also mapcount of subpages
>
> diff --git a/mm/util.c b/mm/util.c
> index 8bf08b5b5760..5c9c7359ee8a 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
> return true;
> if (PageHuge(page))
> return false;
> - for (i = 0; i < hpage_nr_pages(page); i++) {
> + for (i = 0; i < (1 << compound_order(page)); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
> --
> 1.8.3.1
>
--
Michal Hocko
SUSE Labs
On Fri, Nov 30, 2018 at 01:18:51PM +0100, Michal Hocko wrote:
> On Fri 30-11-18 13:06:57, Jan Stancek wrote:
> > LTP proc01 testcase has been observed to rarely trigger crashes
> > on arm64:
> > page_mapped+0x78/0xb4
> > stable_page_flags+0x27c/0x338
> > kpageflags_read+0xfc/0x164
> > proc_reg_read+0x7c/0xb8
> > __vfs_read+0x58/0x178
> > vfs_read+0x90/0x14c
> > SyS_read+0x60/0xc0
> >
> > Issue is that page_mapped() assumes that if compound page is not
> > huge, then it must be THP. But if this is 'normal' compound page
> > (COMPOUND_PAGE_DTOR), then following loop can keep running
> > (for HPAGE_PMD_NR iterations) until it tries to read from memory
> > that isn't mapped and triggers a panic:
> > for (i = 0; i < hpage_nr_pages(page); i++) {
> > if (atomic_read(&page[i]._mapcount) >= 0)
> > return true;
> > }
> >
> > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> > with a custom kernel module [1] which:
> > - allocates compound page (PAGEC) of order 1
> > - allocates 2 normal pages (COPY), which are initialized to 0xff
> > (to satisfy _mapcount >= 0)
> > - 2 PAGEC page structs are copied to address of first COPY page
> > - second page of COPY is marked as not present
> > - call to page_mapped(COPY) now triggers fault on access to 2nd
> > COPY page at offset 0x30 (_mapcount)
> >
> > [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
> >
> > Fix the loop to iterate for "1 << compound_order" pages.
>
> This is much less magic than the previous version. It is still not clear
> to me how is mapping higher order pages to page tables other than THP
> though. So a more detailed information about the source would bre really
> welcome. Once we know that we can add a Fixes tag and also mark the
> patch for stable because that sounds like a stable material.
IIRC, sound subsystem can producuce custom mapped compound pages.
The bug dates back to e1534ae95004.
> > Debugged-by: Laszlo Ersek <[email protected]>
> > Suggested-by: "Kirill A. Shutemov" <[email protected]>
> > Signed-off-by: Jan Stancek <[email protected]>
>
> The patch looks sensible to me
> Acked-by: Michal Hocko <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
--
Kirill A. Shutemov
On Fri 30-11-18 15:36:51, Kirill A. Shutemov wrote:
> On Fri, Nov 30, 2018 at 01:18:51PM +0100, Michal Hocko wrote:
> > On Fri 30-11-18 13:06:57, Jan Stancek wrote:
> > > LTP proc01 testcase has been observed to rarely trigger crashes
> > > on arm64:
> > > page_mapped+0x78/0xb4
> > > stable_page_flags+0x27c/0x338
> > > kpageflags_read+0xfc/0x164
> > > proc_reg_read+0x7c/0xb8
> > > __vfs_read+0x58/0x178
> > > vfs_read+0x90/0x14c
> > > SyS_read+0x60/0xc0
> > >
> > > Issue is that page_mapped() assumes that if compound page is not
> > > huge, then it must be THP. But if this is 'normal' compound page
> > > (COMPOUND_PAGE_DTOR), then following loop can keep running
> > > (for HPAGE_PMD_NR iterations) until it tries to read from memory
> > > that isn't mapped and triggers a panic:
> > > for (i = 0; i < hpage_nr_pages(page); i++) {
> > > if (atomic_read(&page[i]._mapcount) >= 0)
> > > return true;
> > > }
> > >
> > > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> > > with a custom kernel module [1] which:
> > > - allocates compound page (PAGEC) of order 1
> > > - allocates 2 normal pages (COPY), which are initialized to 0xff
> > > (to satisfy _mapcount >= 0)
> > > - 2 PAGEC page structs are copied to address of first COPY page
> > > - second page of COPY is marked as not present
> > > - call to page_mapped(COPY) now triggers fault on access to 2nd
> > > COPY page at offset 0x30 (_mapcount)
> > >
> > > [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
> > >
> > > Fix the loop to iterate for "1 << compound_order" pages.
> >
> > This is much less magic than the previous version. It is still not clear
> > to me how is mapping higher order pages to page tables other than THP
> > though. So a more detailed information about the source would bre really
> > welcome. Once we know that we can add a Fixes tag and also mark the
> > patch for stable because that sounds like a stable material.
>
> IIRC, sound subsystem can producuce custom mapped compound pages.
Do I assume correctly that consecutive ptes simply point to subpages?
> The bug dates back to e1534ae95004.
Thanks for the pointer. I thought this was a new and creative usage of
the pte->page mapping but it really looks like your commit just changed
the underlying semantic.
> > > Debugged-by: Laszlo Ersek <[email protected]>
> > > Suggested-by: "Kirill A. Shutemov" <[email protected]>
> > > Signed-off-by: Jan Stancek <[email protected]>
> >
> > The patch looks sensible to me
> > Acked-by: Michal Hocko <[email protected]>
>
> Acked-by: Kirill A. Shutemov <[email protected]>
>
> --
> Kirill A. Shutemov
--
Michal Hocko
SUSE Labs
On Fri, Nov 30, 2018 at 01:45:46PM +0100, Michal Hocko wrote:
> On Fri 30-11-18 15:36:51, Kirill A. Shutemov wrote:
> > On Fri, Nov 30, 2018 at 01:18:51PM +0100, Michal Hocko wrote:
> > > On Fri 30-11-18 13:06:57, Jan Stancek wrote:
> > > > LTP proc01 testcase has been observed to rarely trigger crashes
> > > > on arm64:
> > > > page_mapped+0x78/0xb4
> > > > stable_page_flags+0x27c/0x338
> > > > kpageflags_read+0xfc/0x164
> > > > proc_reg_read+0x7c/0xb8
> > > > __vfs_read+0x58/0x178
> > > > vfs_read+0x90/0x14c
> > > > SyS_read+0x60/0xc0
> > > >
> > > > Issue is that page_mapped() assumes that if compound page is not
> > > > huge, then it must be THP. But if this is 'normal' compound page
> > > > (COMPOUND_PAGE_DTOR), then following loop can keep running
> > > > (for HPAGE_PMD_NR iterations) until it tries to read from memory
> > > > that isn't mapped and triggers a panic:
> > > > for (i = 0; i < hpage_nr_pages(page); i++) {
> > > > if (atomic_read(&page[i]._mapcount) >= 0)
> > > > return true;
> > > > }
> > > >
> > > > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> > > > with a custom kernel module [1] which:
> > > > - allocates compound page (PAGEC) of order 1
> > > > - allocates 2 normal pages (COPY), which are initialized to 0xff
> > > > (to satisfy _mapcount >= 0)
> > > > - 2 PAGEC page structs are copied to address of first COPY page
> > > > - second page of COPY is marked as not present
> > > > - call to page_mapped(COPY) now triggers fault on access to 2nd
> > > > COPY page at offset 0x30 (_mapcount)
> > > >
> > > > [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
> > > >
> > > > Fix the loop to iterate for "1 << compound_order" pages.
> > >
> > > This is much less magic than the previous version. It is still not clear
> > > to me how is mapping higher order pages to page tables other than THP
> > > though. So a more detailed information about the source would bre really
> > > welcome. Once we know that we can add a Fixes tag and also mark the
> > > patch for stable because that sounds like a stable material.
> >
> > IIRC, sound subsystem can producuce custom mapped compound pages.
>
> Do I assume correctly that consecutive ptes simply point to subpages?
Yes. No huge pages there.
--
Kirill A. Shutemov
On 30.11.18 13:06, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running
> (for HPAGE_PMD_NR iterations) until it tries to read from memory
> that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd
> COPY page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> Fix the loop to iterate for "1 << compound_order" pages.
>
> Debugged-by: Laszlo Ersek <[email protected]>
> Suggested-by: "Kirill A. Shutemov" <[email protected]>
> Signed-off-by: Jan Stancek <[email protected]>
> ---
> mm/util.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Changes in v2:
> - change the loop instead so we check also mapcount of subpages
>
> diff --git a/mm/util.c b/mm/util.c
> index 8bf08b5b5760..5c9c7359ee8a 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
> return true;
> if (PageHuge(page))
> return false;
> - for (i = 0; i < hpage_nr_pages(page); i++) {
> + for (i = 0; i < (1 << compound_order(page)); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
Reviewed-by: David Hildenbrand <[email protected]>
--
Thanks,
David / dhildenb
On 11/30/18 13:06, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running
> (for HPAGE_PMD_NR iterations) until it tries to read from memory
> that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd
> COPY page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> Fix the loop to iterate for "1 << compound_order" pages.
>
> Debugged-by: Laszlo Ersek <[email protected]>
> Suggested-by: "Kirill A. Shutemov" <[email protected]>
> Signed-off-by: Jan Stancek <[email protected]>
> ---
> mm/util.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Changes in v2:
> - change the loop instead so we check also mapcount of subpages
>
> diff --git a/mm/util.c b/mm/util.c
> index 8bf08b5b5760..5c9c7359ee8a 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
> return true;
> if (PageHuge(page))
> return false;
> - for (i = 0; i < hpage_nr_pages(page); i++) {
> + for (i = 0; i < (1 << compound_order(page)); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
Totally uninformed side-question:
how large can the return value of compound_order() be? MAX_ORDER?
Apparently, MAX_ORDER can be defined as CONFIG_FORCE_MAX_ZONEORDER.
"config FORCE_MAX_ZONEORDER" is listed in a number of Kconfig files.
Among those, "arch/mips/Kconfig" permits "ranges" (?) that extend up to
64. Same applies to "arch/powerpc/Kconfig" and "arch/sh/mm/Kconfig".
If we left-shift "1" -- a signed int, which I assume in practice will
always have two's complement representation, 1 sign bit, 31 value bits,
and 0 padding bits --, by 31 or more bit positions, we get undefined
behavior (as part of the left-shift operation).
Is this a practical concern?
Thanks,
Laszlo
On Mon, Dec 03, 2018 at 11:23:58AM +0100, Laszlo Ersek wrote:
> Totally uninformed side-question:
>
> how large can the return value of compound_order() be? MAX_ORDER?
>
> Apparently, MAX_ORDER can be defined as CONFIG_FORCE_MAX_ZONEORDER.
>
> "config FORCE_MAX_ZONEORDER" is listed in a number of Kconfig files.
> Among those, "arch/mips/Kconfig" permits "ranges" (?) that extend up to
> 64. Same applies to "arch/powerpc/Kconfig" and "arch/sh/mm/Kconfig".
>
> If we left-shift "1" -- a signed int, which I assume in practice will
> always have two's complement representation, 1 sign bit, 31 value bits,
> and 0 padding bits --, by 31 or more bit positions, we get undefined
> behavior (as part of the left-shift operation).
>
> Is this a practical concern?
Not really.
Assuming 4k PAGE_SIZE, compound_order() == 31 means 8 TiB pages. I doubt
we will see such allocation requests any time soon.
Even with 1k base page size, it's still 2 TiB.
We will see other limitations in page allocaiton path before the compund
order type will be an issue.
--
Kirill A. Shutemov
[ CC'ed Andrew for potential inclusion in -mm ]
On Fri, Nov 30, 2018 at 01:06:57PM +0100, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running
> (for HPAGE_PMD_NR iterations) until it tries to read from memory
> that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd
> COPY page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> Fix the loop to iterate for "1 << compound_order" pages.
>
> Debugged-by: Laszlo Ersek <[email protected]>
> Suggested-by: "Kirill A. Shutemov" <[email protected]>
> Signed-off-by: Jan Stancek <[email protected]>
> ---
> mm/util.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Changes in v2:
> - change the loop instead so we check also mapcount of subpages
Reviewed-by: Andrea Arcangeli <[email protected]>
Thanks,
Andrea
On Fri, Nov 30, 2018 at 1:07 PM Jan Stancek <[email protected]> wrote:
>
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running
> (for HPAGE_PMD_NR iterations) until it tries to read from memory
> that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd
> COPY page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> Fix the loop to iterate for "1 << compound_order" pages.
>
> Debugged-by: Laszlo Ersek <[email protected]>
> Suggested-by: "Kirill A. Shutemov" <[email protected]>
> Signed-off-by: Jan Stancek <[email protected]>
> ---
> mm/util.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Changes in v2:
> - change the loop instead so we check also mapcount of subpages
>
> diff --git a/mm/util.c b/mm/util.c
> index 8bf08b5b5760..5c9c7359ee8a 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
> return true;
> if (PageHuge(page))
> return false;
> - for (i = 0; i < hpage_nr_pages(page); i++) {
> + for (i = 0; i < (1 << compound_order(page)); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
> --
> 1.8.3.1
Hi all
This patch landed in the 4.9-stable tree starting from 4.9.151 and it
broke our MIPS1004kc system with CONFIG_HIGHMEM=y.
The breakage consists of random processes dying with SIGILL or SIGSEGV
when we stress test the system with high memory pressure and explicit
memory compaction requested through /proc/sys/vm/compact_memory.
Reverting this patch fixes the crashes.
We can put some effort on debugging if there are no obvious
explanations for this. Keep in mind that this is 32-bit system with
HIGHMEM.
BR,
Lars
----- Original Message -----
> On Fri, Nov 30, 2018 at 1:07 PM Jan Stancek <[email protected]> wrote:
> >
> > LTP proc01 testcase has been observed to rarely trigger crashes
> > on arm64:
> > page_mapped+0x78/0xb4
> > stable_page_flags+0x27c/0x338
> > kpageflags_read+0xfc/0x164
> > proc_reg_read+0x7c/0xb8
> > __vfs_read+0x58/0x178
> > vfs_read+0x90/0x14c
> > SyS_read+0x60/0xc0
> >
> > Issue is that page_mapped() assumes that if compound page is not
> > huge, then it must be THP. But if this is 'normal' compound page
> > (COMPOUND_PAGE_DTOR), then following loop can keep running
> > (for HPAGE_PMD_NR iterations) until it tries to read from memory
> > that isn't mapped and triggers a panic:
> > for (i = 0; i < hpage_nr_pages(page); i++) {
> > if (atomic_read(&page[i]._mapcount) >= 0)
> > return true;
> > }
> >
> > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> > with a custom kernel module [1] which:
> > - allocates compound page (PAGEC) of order 1
> > - allocates 2 normal pages (COPY), which are initialized to 0xff
> > (to satisfy _mapcount >= 0)
> > - 2 PAGEC page structs are copied to address of first COPY page
> > - second page of COPY is marked as not present
> > - call to page_mapped(COPY) now triggers fault on access to 2nd
> > COPY page at offset 0x30 (_mapcount)
> >
> > [1]
> > https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
> >
> > Fix the loop to iterate for "1 << compound_order" pages.
> >
> > Debugged-by: Laszlo Ersek <[email protected]>
> > Suggested-by: "Kirill A. Shutemov" <[email protected]>
> > Signed-off-by: Jan Stancek <[email protected]>
> > ---
> > mm/util.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > Changes in v2:
> > - change the loop instead so we check also mapcount of subpages
> >
> > diff --git a/mm/util.c b/mm/util.c
> > index 8bf08b5b5760..5c9c7359ee8a 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
> > return true;
> > if (PageHuge(page))
> > return false;
> > - for (i = 0; i < hpage_nr_pages(page); i++) {
> > + for (i = 0; i < (1 << compound_order(page)); i++) {
> > if (atomic_read(&page[i]._mapcount) >= 0)
> > return true;
> > }
> > --
> > 1.8.3.1
>
> Hi all
>
> This patch landed in the 4.9-stable tree starting from 4.9.151 and it
> broke our MIPS1004kc system with CONFIG_HIGHMEM=y.
Hi,
are you using THP (CONFIG_TRANSPARENT_HUGEPAGE)?
The changed line should affect only THP and normal compound pages,
so a test with THP disabled might be interesting.
>
> The breakage consists of random processes dying with SIGILL or SIGSEGV
> when we stress test the system with high memory pressure and explicit
> memory compaction requested through /proc/sys/vm/compact_memory.
> Reverting this patch fixes the crashes.
>
> We can put some effort on debugging if there are no obvious
> explanations for this. Keep in mind that this is 32-bit system with
> HIGHMEM.
Nothing obvious that I can see. I've been trying to reproduce on
32-bit x86 Fedora with no luck so far.
Regards,
Jan
On Tue, Feb 5, 2019 at 8:14 AM Jan Stancek <[email protected]> wrote:
> Hi,
>
> are you using THP (CONFIG_TRANSPARENT_HUGEPAGE)?
>
> The changed line should affect only THP and normal compound pages,
> so a test with THP disabled might be interesting.
>
> >
> > The breakage consists of random processes dying with SIGILL or SIGSEGV
> > when we stress test the system with high memory pressure and explicit
> > memory compaction requested through /proc/sys/vm/compact_memory.
> > Reverting this patch fixes the crashes.
> >
> > We can put some effort on debugging if there are no obvious
> > explanations for this. Keep in mind that this is 32-bit system with
> > HIGHMEM.
>
> Nothing obvious that I can see. I've been trying to reproduce on
> 32-bit x86 Fedora with no luck so far.
>
Hi
Thanks for looking in to it. After some deep dive in MM code, I think
it is safe to say this patch was innocent.
All traces studied so far points to a missing cache coherency call in
mm/migrate.c:migrate_page that is needed only for those evil MIPSes
that lack I/D cache coherency. I will send a write-up to linux-mips
about this. Basically for a non-mapped page it does only a copy of
page data and metadata but no flush_dcache_page() call will be done.
This races with subsequent use of the page.
BR,
Lars
On Mon 18-02-19 14:43:58, Lars Persson wrote:
> On Tue, Feb 5, 2019 at 8:14 AM Jan Stancek <[email protected]> wrote:
> > Hi,
> >
> > are you using THP (CONFIG_TRANSPARENT_HUGEPAGE)?
> >
> > The changed line should affect only THP and normal compound pages,
> > so a test with THP disabled might be interesting.
> >
> > >
> > > The breakage consists of random processes dying with SIGILL or SIGSEGV
> > > when we stress test the system with high memory pressure and explicit
> > > memory compaction requested through /proc/sys/vm/compact_memory.
> > > Reverting this patch fixes the crashes.
> > >
> > > We can put some effort on debugging if there are no obvious
> > > explanations for this. Keep in mind that this is 32-bit system with
> > > HIGHMEM.
> >
> > Nothing obvious that I can see. I've been trying to reproduce on
> > 32-bit x86 Fedora with no luck so far.
> >
>
> Hi
>
> Thanks for looking in to it. After some deep dive in MM code, I think
> it is safe to say this patch was innocent.
>
> All traces studied so far points to a missing cache coherency call in
> mm/migrate.c:migrate_page that is needed only for those evil MIPSes
> that lack I/D cache coherency. I will send a write-up to linux-mips
> about this. Basically for a non-mapped page it does only a copy of
> page data and metadata but no flush_dcache_page() call will be done.
> This races with subsequent use of the page.
Please make sure to cc linux-mm for the patch
--
Michal Hocko
SUSE Labs