Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP partially

[Yang Shi]

On 1/14/20 11:28 AM, Yang Shi wrote:
> On 12/4/19 4:15 PM, Hugh Dickins wrote:
> > On Wed, 4 Dec 2019, Yang Shi wrote:
> >
> > > Currently when truncating shmem file, if the range is partial of THP
> > > (start or end is in the middle of THP), the pages actually will just 
> > > get
> > > cleared rather than being freed unless the range cover the whole THP.
> > > Even though all the subpages are truncated (randomly or sequentially),
> > > the THP may still be kept in page cache. This might be fine for some
> > > usecases which prefer preserving THP.
> > >
> > > But, when doing balloon inflation in QEMU, QEMU actually does hole 
> > > punch
> > > or MADV_DONTNEED in base page size granulairty if hugetlbfs is not 
> > > used.
> > > So, when using shmem THP as memory backend QEMU inflation actually 
> > > doesn't
> > > work as expected since it doesn't free memory. But, the inflation
> > > usecase really needs get the memory freed. Anonymous THP will not get
> > > freed right away too but it will be freed eventually when all 
> > > subpages are
> > > unmapped, but shmem THP would still stay in page cache.
> > >
> > > Split THP right away when doing partial hole punch, and if split fails
> > > just clear the page so that read to the hole punched area would return
> > > zero.
> > >
> > > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > > ---
> > > v2: * Adopted the comment from Kirill.
> > > ÂÂÂÂ * Dropped fallocate mode flag, THP split is the default behavior.
> > > ÂÂÂÂ * Blended Huge's implementation with my v1 patch. TBH I'm not 
> > > very keen to
> > > ÂÂÂÂÂÂ Hugh's find_get_entries() hack (basically neutral), but 
> > > without that hack
> > Thanks for giving it a try. I'm not neutral about my find_get_entries()
> > hack: it surely had to go (without it, I'd have just pushed my own 
> > patch).
> > I've not noticed anything wrong with your patch, and it's in the right
> > direction, but I'm still not thrilled with it. I also remember that I
> > got the looping wrong in my first internal attempt (fixed in what I 
> > sent),
> > and need to be very sure of the try-again-versus-move-on-to-next 
> > conditions
> > before agreeing to anything. No rush, I'll come back to this in days or
> > month ahead: I'll try to find a less gotoey blend of yours and mine.
>
> Hi Hugh,
>
> Any update on this one?
>
> Thanks,
> Yang

Hi Hugh,

Ping. Any comment on this? I really hope it can make v5.7.

Thanks,
Yang

> > Hugh
> >
> > > ÂÂÂÂÂÂ we have to rely on pagevec_release() to release extra pins 
> > > and play with
> > > ÂÂÂÂÂÂ goto. This version does in this way. The patch is bigger than 
> > > Hugh's due
> > > ÂÂÂÂÂÂ to extra comments to make the flow clear.
> > >
> > > Â mm/shmem.c | 120 
> > > ++++++++++++++++++++++++++++++++++++++++++-------------------
> > > Â 1 file changed, 83 insertions(+), 37 deletions(-)
> > >
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 220be9f..1ae0c7f 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -806,12 +806,15 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂ long nr_swaps_freed = 0;
> > > ÂÂÂÂÂ pgoff_t index;
> > > ÂÂÂÂÂ int i;
> > > +ÂÂÂ bool split = false;
> > > +ÂÂÂ struct page *page = NULL;
> > > Â ÂÂÂÂÂ if (lend == -1)
> > > ÂÂÂÂÂÂÂÂÂ end = -1;ÂÂÂ /* unsigned, so actually very big */
> > > Â ÂÂÂÂÂ pagevec_init(&pvec);
> > > ÂÂÂÂÂ index = start;
> > > +retry:
> > > ÂÂÂÂÂ while (index < end) {
> > > ÂÂÂÂÂÂÂÂÂ pvec.nr = find_get_entries(mapping, index,
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ min(end - index, (pgoff_t)PAGEVEC_SIZE),
> > > @@ -819,7 +822,8 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂÂÂÂÂ if (!pvec.nr)
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
> > > ÂÂÂÂÂÂÂÂÂ for (i = 0; i < pagevec_count(&pvec); i++) {
> > > -ÂÂÂÂÂÂÂÂÂÂÂ struct page *page = pvec.pages[i];
> > > +ÂÂÂÂÂÂÂÂÂÂÂ split = false;
> > > +ÂÂÂÂÂÂÂÂÂÂÂ page = pvec.pages[i];
> > > Â ÂÂÂÂÂÂÂÂÂÂÂÂÂ index = indices[i];
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (index >= end)
> > > @@ -838,23 +842,24 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!trylock_page(page))
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > Â -ÂÂÂÂÂÂÂÂÂÂÂ if (PageTransTail(page)) {
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* Middle of THP: zero out the page */
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ clear_highpage(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > -ÂÂÂÂÂÂÂÂÂÂÂ } else if (PageTransHuge(page)) {
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (index == round_down(end, HPAGE_PMD_NR)) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ if (PageTransCompound(page) && !unfalloc) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (PageHead(page) &&
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ index != round_down(end, HPAGE_PMD_NR)) {
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Range ends in the middle of THP:
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * zero out the page
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Fall through when punching whole
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * THP.
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ clear_highpage(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ index += HPAGE_PMD_NR - 1;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ i += HPAGE_PMD_NR - 1;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ } else {
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Split THP for any partial hole
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * punch.
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ get_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ split = true;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto split;
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ }
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ index += HPAGE_PMD_NR - 1;
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ i += HPAGE_PMD_NR - 1;
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ }
> > > Â ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!unfalloc || !PageUptodate(page)) {
> > > @@ -866,9 +871,29 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ }
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > ÂÂÂÂÂÂÂÂÂ }
> > > +split:
> > > ÂÂÂÂÂÂÂÂÂ pagevec_remove_exceptionals(&pvec);
> > > ÂÂÂÂÂÂÂÂÂ pagevec_release(&pvec);
> > > ÂÂÂÂÂÂÂÂÂ cond_resched();
> > > +
> > > +ÂÂÂÂÂÂÂ if (split) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ /*
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ * The pagevec_release() released all extra pins
> > > + * from pagevec lookup. And we hold an extra pin
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ * and still have the page locked under us.
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > +ÂÂÂÂÂÂÂÂÂÂÂ if (!split_huge_page(page)) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ put_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* Re-lookup page cache from current index */
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto retry;
> > > +ÂÂÂÂÂÂÂÂÂÂÂ }
> > > +
> > > +ÂÂÂÂÂÂÂÂÂÂÂ /* Fail to split THP, move to next index */
> > > +ÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂ put_page(page);
> > > +ÂÂÂÂÂÂÂ }
> > > +
> > > ÂÂÂÂÂÂÂÂÂ index++;
> > > ÂÂÂÂÂ }
> > > Â @@ -901,6 +926,7 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂÂÂÂÂ return;
> > > Â ÂÂÂÂÂ index = start;
> > > +again:
> > > ÂÂÂÂÂ while (index < end) {
> > > ÂÂÂÂÂÂÂÂÂ cond_resched();
> > > Â @@ -916,7 +942,8 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > ÂÂÂÂÂÂÂÂÂ }
> > > ÂÂÂÂÂÂÂÂÂ for (i = 0; i < pagevec_count(&pvec); i++) {
> > > -ÂÂÂÂÂÂÂÂÂÂÂ struct page *page = pvec.pages[i];
> > > +ÂÂÂÂÂÂÂÂÂÂÂ split = false;
> > > +ÂÂÂÂÂÂÂÂÂÂÂ page = pvec.pages[i];
> > > Â ÂÂÂÂÂÂÂÂÂÂÂÂÂ index = indices[i];
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (index >= end)
> > > @@ -936,30 +963,24 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > Â ÂÂÂÂÂÂÂÂÂÂÂÂÂ lock_page(page);
> > > Â -ÂÂÂÂÂÂÂÂÂÂÂ if (PageTransTail(page)) {
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* Middle of THP: zero out the page */
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ clear_highpage(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Partial thp truncate due 'start' in middle
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * of THP: don't need to look on these pages
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * again on !pvec.nr restart.
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (index != round_down(end, HPAGE_PMD_NR))
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ start++;
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > -ÂÂÂÂÂÂÂÂÂÂÂ } else if (PageTransHuge(page)) {
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (index == round_down(end, HPAGE_PMD_NR)) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ if (PageTransCompound(page) && !unfalloc) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (PageHead(page) &&
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ index != round_down(end, HPAGE_PMD_NR)) {
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Range ends in the middle of THP:
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * zero out the page
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Fall through when punching whole
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * THP.
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ clear_highpage(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ index += HPAGE_PMD_NR - 1;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ i += HPAGE_PMD_NR - 1;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ } else {
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Split THP for any partial hole
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * punch.
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ get_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ split = true;
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto rescan_split;
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ }
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ index += HPAGE_PMD_NR - 1;
> > > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ i += HPAGE_PMD_NR - 1;
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ }
> > > Â ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!unfalloc || !PageUptodate(page)) {
> > > @@ -976,8 +997,33 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ }
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > ÂÂÂÂÂÂÂÂÂ }
> > > +rescan_split:
> > > ÂÂÂÂÂÂÂÂÂ pagevec_remove_exceptionals(&pvec);
> > > ÂÂÂÂÂÂÂÂÂ pagevec_release(&pvec);
> > > +
> > > +ÂÂÂÂÂÂÂ if (split) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ /*
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ * The pagevec_release() released all extra pins
> > > + * from pagevec lookup. And we hold an extra pin
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ * and still have the page locked under us.
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > +ÂÂÂÂÂÂÂÂÂÂÂ if (!split_huge_page(page)) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ put_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* Re-lookup page cache from current index */
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto again;
> > > +ÂÂÂÂÂÂÂÂÂÂÂ }
> > > +
> > > +ÂÂÂÂÂÂÂÂÂÂÂ /*
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ * Split fail, clear the page then move to next
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ * index.
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > +ÂÂÂÂÂÂÂÂÂÂÂ clear_highpage(page);
> > > +
> > > +ÂÂÂÂÂÂÂÂÂÂÂ unlock_page(page);
> > > +ÂÂÂÂÂÂÂÂÂÂÂ put_page(page);
> > > +ÂÂÂÂÂÂÂ }
> > > +
> > > ÂÂÂÂÂÂÂÂÂ index++;
> > > ÂÂÂÂÂ }
> > > Â --
> > > 1.8.3.1