by Minchan Kim

[permalink] [raw]

Subject: Re: [PATCH v10 1/7] mm: support madvise(MADV_FREE)

On Mon, Jul 07, 2014 at 01:41:37PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jul 07, 2014 at 09:53:52AM +0900, Minchan Kim wrote:
> > Linux doesn't have an ability to free pages lazy while other OS
> > already have been supported that named by madvise(MADV_FREE).
> >
> > The gain is clear that kernel can discard freed pages rather than
> > swapping out or OOM if memory pressure happens.
> >
> > Without memory pressure, freed pages would be reused by userspace
> > without another additional overhead(ex, page fault + allocation
> > + zeroing).
> >
> > How to work is following as.
> >
> > When madvise syscall is called, VM clears dirty bit of ptes of
> > the range. If memory pressure happens, VM checks dirty bit of
> > page table and if it found still "clean", it means it's a
> > "lazyfree pages" so VM could discard the page instead of swapping out.
> > Once there was store operation for the page before VM peek a page
> > to reclaim, dirty bit is set so VM can swap out the page instead of
> > discarding.
> >
> > Firstly, heavy users would be general allocators(ex, jemalloc,
> > tcmalloc and hope glibc supports it) and jemalloc/tcmalloc already
> > have supported the feature for other OS(ex, FreeBSD)
> >
> > barrios@blaptop:~/benchmark/ebizzy$ lscpu
> > Architecture: x86_64
> > CPU op-mode(s): 32-bit, 64-bit
> > Byte Order: Little Endian
> > CPU(s): 4
> > On-line CPU(s) list: 0-3
> > Thread(s) per core: 2
> > Core(s) per socket: 2
> > Socket(s): 1
> > NUMA node(s): 1
> > Vendor ID: GenuineIntel
> > CPU family: 6
> > Model: 42
> > Stepping: 7
> > CPU MHz: 2801.000
> > BogoMIPS: 5581.64
> > Virtualization: VT-x
> > L1d cache: 32K
> > L1i cache: 32K
> > L2 cache: 256K
> > L3 cache: 4096K
> > NUMA node0 CPU(s): 0-3
> >
> > ebizzy benchmark(./ebizzy -S 10 -n 512)
> >
> > vanilla-jemalloc MADV_free-jemalloc
> >
> > 1 thread
> > records: 10 records: 10
> > avg: 7682.10 avg: 15306.10
> > std: 62.35(0.81%) std: 347.99(2.27%)
> > max: 7770.00 max: 15622.00
> > min: 7598.00 min: 14772.00
> >
> > 2 thread
> > records: 10 records: 10
> > avg: 12747.50 avg: 24171.00
> > std: 792.06(6.21%) std: 895.18(3.70%)
> > max: 13337.00 max: 26023.00
> > min: 10535.00 min: 23152.00
> >
> > 4 thread
> > records: 10 records: 10
> > avg: 16474.60 avg: 33717.90
> > std: 1496.45(9.08%) std: 2008.97(5.96%)
> > max: 17877.00 max: 35958.00
> > min: 12224.00 min: 29565.00
> >
> > 8 thread
> > records: 10 records: 10
> > avg: 16778.50 avg: 33308.10
> > std: 825.53(4.92%) std: 1668.30(5.01%)
> > max: 17543.00 max: 36010.00
> > min: 14576.00 min: 29577.00
> >
> > 16 thread
> > records: 10 records: 10
> > avg: 20614.40 avg: 35516.30
> > std: 602.95(2.92%) std: 1283.65(3.61%)
> > max: 21753.00 max: 37178.00
> > min: 19605.00 min: 33217.00
> >
> > 32 thread
> > records: 10 records: 10
> > avg: 22771.70 avg: 36018.50
> > std: 598.94(2.63%) std: 1046.76(2.91%)
> > max: 24035.00 max: 37266.00
> > min: 22108.00 min: 34149.00
> >
> > In summary, MADV_FREE is about 2 time faster than MADV_DONTNEED.
> >
> > Cc: Michael Kerrisk <[email protected]>
> > Cc: Linux API <[email protected]>
> > Cc: Hugh Dickins <[email protected]>
> > Cc: Johannes Weiner <[email protected]>
> > Cc: KOSAKI Motohiro <[email protected]>
> > Cc: Mel Gorman <[email protected]>
> > Cc: Jason Evans <[email protected]>
> > Cc: Zhang Yanfei <[email protected]>
> > Acked-by: Rik van Riel <[email protected]>
> > Signed-off-by: Minchan Kim <[email protected]>
> > ---
>
> ...
>
> > +static void madvise_free_page_range(struct mmu_gather *tlb,
> > + struct vm_area_struct *vma,
> > + unsigned long addr, unsigned long end)
> > +{
> > + pgd_t *pgd;
> > + unsigned long next;
> > +
> > + BUG_ON(addr >= end);
> > + tlb_start_vma(tlb, vma);
> > + pgd = pgd_offset(vma->vm_mm, addr);
> > + do {
> > + next = pgd_addr_end(addr, end);
> > + if (pgd_none_or_clear_bad(pgd))
> > + continue;
> > + next = madvise_free_pud_range(tlb, vma, pgd, addr, next);
> > + } while (pgd++, addr = next, addr != end);
> > + tlb_end_vma(tlb, vma);
>
> Any particular reason why pagewalker can't be used here?

Nothing special. I just copied from MADV_DONTNEED.
Will try it.

>
> > +}
>
> ...
>
> > @@ -381,6 +547,13 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
> > return madvise_remove(vma, prev, start, end);
> > case MADV_WILLNEED:
> > return madvise_willneed(vma, prev, start, end);
> > + case MADV_FREE:
> > + /*
> > + * XXX: In this implementation, MADV_FREE works like
> > + * MADV_DONTNEED on swapless system or full swap.
> > + */
> > + if (get_nr_swap_pages() > 0)
> > + return madvise_free(vma, prev, start, end);
>
> Looks racy wrt to full swap. What will happen if we will do madvise_free()
> on full swap?

Now, we don't age anonymous LRU list if swap is full so that VM would lose
the chance to discard freed page via shrink_page_list in this implementation
if that race hppanes.

But it would be not severe because MADV_FREE semantic doesn't say VM must
discard them but it is just hint from userside that specified range is
no longer important so that VM can gave a freedom to free and I think
it's not a common case.

In addition, I have a plan to support MADV_FREE on swapless system, too.

>
> > case MADV_DONTNEED:
> > return madvise_dontneed(vma, prev, start, end);
> > default:
>
> ...
>
> > @@ -1204,6 +1223,16 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> > }
> > dec_mm_counter(mm, MM_ANONPAGES);
> > inc_mm_counter(mm, MM_SWAPENTS);
> > + } else if (flags & TTU_UNMAP) {
> > + if (dirty || PageDirty(page)) {
> > + set_pte_at(mm, address, pte, pteval);
> > + ret = SWAP_FAIL;
> > + goto out_unmap;
>
> I don't get this part.
> Looks like it will fail to unmap the page if it's dirty and not backed by
> swapcache. Current code doesn't have such limitation.
> Do we really need this?

Good point. Code is rather ugly even, it has side-effect with hwpoisend
page unmapping.

How about this? I didn't test it but if there is no objection,
I will go this with stress testing.

---
include/linux/rmap.h | 1 +
mm/rmap.c | 22 ++++++++++++----------
mm/vmscan.c | 5 +++--
3 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index dea05914f167..0ba377b97a38 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -75,6 +75,7 @@ enum ttu_flags {
TTU_UNMAP = 1, /* unmap mode */
TTU_MIGRATION = 2, /* migration mode */
TTU_MUNLOCK = 4, /* munlock mode */
+ TTU_FREE = 8, /* free mode */

TTU_IGNORE_MLOCK = (1 << 8), /* ignore mlock */
TTU_IGNORE_ACCESS = (1 << 9), /* don't age */
diff --git a/mm/rmap.c b/mm/rmap.c
index 3c415eb8b6f0..010d51ea26c4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1209,6 +1209,18 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
swp_entry_t entry = { .val = page_private(page) };
pte_t swp_pte;

+ if (flags & TTU_FREE) {
+ if (dirty || PageDirty(page)) {
+ set_pte_at(mm, address, pte, pteval);
+ ret = SWAP_FAIL;
+ goto out_unmap;
+ } else {
+ /* It's a freeable page by MADV_FREE */
+ dec_mm_counter(mm, MM_ANONPAGES);
+ goto discard;
+ }
+ }
+
if (PageSwapCache(page)) {
/*
* Store the swap location in the pte.
@@ -1227,16 +1239,6 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
}
dec_mm_counter(mm, MM_ANONPAGES);
inc_mm_counter(mm, MM_SWAPENTS);
- } else if (flags & TTU_UNMAP) {
- if (dirty || PageDirty(page)) {
- set_pte_at(mm, address, pte, pteval);
- ret = SWAP_FAIL;
- goto out_unmap;
- } else {
- /* It's a freeable page by madvise_free */
- dec_mm_counter(mm, MM_ANONPAGES);
- goto discard;
- }
} else if (IS_ENABLED(CONFIG_MIGRATION)) {
/*
* Store the pfn of the page in a special migration
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4e15babf4414..a7dbce703208 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1549,8 +1549,9 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
if (nr_taken == 0)
return 0;

- nr_reclaimed = shrink_page_list(&page_list, zone, sc, TTU_UNMAP,
- &nr_dirty, &nr_unqueued_dirty, &nr_congested,
+ nr_reclaimed = shrink_page_list(&page_list, zone, sc,
+ TTU_UNMAP|TTU_FREE, &nr_dirty,
+ &nr_unqueued_dirty, &nr_congested,
&nr_writeback, &nr_immediate,
false);

--
2.0.0

>
> > + } else {
> > + /* It's a freeable page by madvise_free */
> > + dec_mm_counter(mm, MM_ANONPAGES);
> > + goto discard;
> > + }
> > } else if (IS_ENABLED(CONFIG_MIGRATION)) {
> > /*
> > * Store the pfn of the page in a special migration
>
> --
> Kirill A. Shutemov
>
> --
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--
Kind regards,
Minchan Kim

2014-07-08 01:09:40

On Tue, Jul 08, 2014 at 10:30:38AM +0900, Minchan Kim wrote:
> Actually, I did but found no problem except CONFIG_DEBUG_VM but rollback
> after peeking [1].
> When I read the description in detail by your review, I think we can remove
> BUG_ON(PageTransHuge(page)) in try_to_unmap and go with no split for lazyfree
> page because they are not in swapcache any more so the assumption of [1] is
> not valid. Will do it in next revision.

No. try_to_unmap() knows nothing about PMDs, so page_check_address() will
always return NULL for THP and you will not unmap anything.

--
Kirill A. Shutemov