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

[Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>]

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.
> > >      * Dropped fallocate mode flag, THP split is the default behavior.
> > >      * Blended Huge's implementation with my v1 patch. TBH I'm not 
> > > very keen to
> > >        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
> >
> > >        we have to rely on pagevec_release() to release extra pins 
> > > and play with
> > >        goto. This version does in this way. The patch is bigger than 
> > > Hugh's due
> > >        to extra comments to make the flow clear.
> > >
> > >   mm/shmem.c | 120 
> > > ++++++++++++++++++++++++++++++++++++++++++-------------------
> > >   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,
> > >       long nr_swaps_freed = 0;
> > >       pgoff_t index;
> > >       int i;
> > > +    bool split = false;
> > > +    struct page *page = NULL;
> > >         if (lend == -1)
> > >           end = -1;    /* unsigned, so actually very big */
> > >         pagevec_init(&pvec);
> > >       index = start;
> > > +retry:
> > >       while (index < end) {
> > >           pvec.nr = find_get_entries(mapping, index,
> > >               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,
> > >           if (!pvec.nr)
> > >               break;
> > >           for (i = 0; i < pagevec_count(&pvec); i++) {
> > > -            struct page *page = pvec.pages[i];
> > > +            split = false;
> > > +            page = pvec.pages[i];
> > >                 index = indices[i];
> > >               if (index >= end)
> > > @@ -838,23 +842,24 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > >               if (!trylock_page(page))
> > >                   continue;
> > >   -            if (PageTransTail(page)) {
> > > -                /* Middle of THP: zero out the page */
> > > -                clear_highpage(page);
> > > -                unlock_page(page);
> > > -                continue;
> > > -            } else if (PageTransHuge(page)) {
> > > -                if (index == round_down(end, HPAGE_PMD_NR)) {
> > > +            if (PageTransCompound(page) && !unfalloc) {
> > > +                if (PageHead(page) &&
> > > +                    index != round_down(end, HPAGE_PMD_NR)) {
> > >                       /*
> > > -                     * Range ends in the middle of THP:
> > > -                     * zero out the page
> > > +                     * Fall through when punching whole
> > > +                     * THP.
> > >                        */
> > > -                    clear_highpage(page);
> > > -                    unlock_page(page);
> > > -                    continue;
> > > +                    index += HPAGE_PMD_NR - 1;
> > > +                    i += HPAGE_PMD_NR - 1;
> > > +                } else {
> > > +                    /*
> > > +                     * Split THP for any partial hole
> > > +                     * punch.
> > > +                     */
> > > +                    get_page(page);
> > > +                    split = true;
> > > +                    goto split;
> > >                   }
> > > -                index += HPAGE_PMD_NR - 1;
> > > -                i += HPAGE_PMD_NR - 1;
> > >               }
> > >                 if (!unfalloc || !PageUptodate(page)) {
> > > @@ -866,9 +871,29 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > >               }
> > >               unlock_page(page);
> > >           }
> > > +split:
> > >           pagevec_remove_exceptionals(&pvec);
> > >           pagevec_release(&pvec);
> > >           cond_resched();
> > > +
> > > +        if (split) {
> > > +            /*
> > > +             * The pagevec_release() released all extra pins
> > > +             * from pagevec lookup.  And we hold an extra pin
> > > +             * and still have the page locked under us.
> > > +             */
> > > +            if (!split_huge_page(page)) {
> > > +                unlock_page(page);
> > > +                put_page(page);
> > > +                /* Re-lookup page cache from current index */
> > > +                goto retry;
> > > +            }
> > > +
> > > +            /* Fail to split THP, move to next index */
> > > +            unlock_page(page);
> > > +            put_page(page);
> > > +        }
> > > +
> > >           index++;
> > >       }
> > >   @@ -901,6 +926,7 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > >           return;
> > >         index = start;
> > > +again:
> > >       while (index < end) {
> > >           cond_resched();
> > >   @@ -916,7 +942,8 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > >               continue;
> > >           }
> > >           for (i = 0; i < pagevec_count(&pvec); i++) {
> > > -            struct page *page = pvec.pages[i];
> > > +            split = false;
> > > +            page = pvec.pages[i];
> > >                 index = indices[i];
> > >               if (index >= end)
> > > @@ -936,30 +963,24 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > >                 lock_page(page);
> > >   -            if (PageTransTail(page)) {
> > > -                /* Middle of THP: zero out the page */
> > > -                clear_highpage(page);
> > > -                unlock_page(page);
> > > -                /*
> > > -                 * Partial thp truncate due 'start' in middle
> > > -                 * of THP: don't need to look on these pages
> > > -                 * again on !pvec.nr restart.
> > > -                 */
> > > -                if (index != round_down(end, HPAGE_PMD_NR))
> > > -                    start++;
> > > -                continue;
> > > -            } else if (PageTransHuge(page)) {
> > > -                if (index == round_down(end, HPAGE_PMD_NR)) {
> > > +            if (PageTransCompound(page) && !unfalloc) {
> > > +                if (PageHead(page) &&
> > > +                    index != round_down(end, HPAGE_PMD_NR)) {
> > >                       /*
> > > -                     * Range ends in the middle of THP:
> > > -                     * zero out the page
> > > +                     * Fall through when punching whole
> > > +                     * THP.
> > >                        */
> > > -                    clear_highpage(page);
> > > -                    unlock_page(page);
> > > -                    continue;
> > > +                    index += HPAGE_PMD_NR - 1;
> > > +                    i += HPAGE_PMD_NR - 1;
> > > +                } else {
> > > +                    /*
> > > +                     * Split THP for any partial hole
> > > +                     * punch.
> > > +                     */
> > > +                    get_page(page);
> > > +                    split = true;
> > > +                    goto rescan_split;
> > >                   }
> > > -                index += HPAGE_PMD_NR - 1;
> > > -                i += HPAGE_PMD_NR - 1;
> > >               }
> > >                 if (!unfalloc || !PageUptodate(page)) {
> > > @@ -976,8 +997,33 @@ static void shmem_undo_range(struct inode 
> > > *inode, loff_t lstart, loff_t lend,
> > >               }
> > >               unlock_page(page);
> > >           }
> > > +rescan_split:
> > >           pagevec_remove_exceptionals(&pvec);
> > >           pagevec_release(&pvec);
> > > +
> > > +        if (split) {
> > > +            /*
> > > +             * The pagevec_release() released all extra pins
> > > +             * from pagevec lookup.  And we hold an extra pin
> > > +             * and still have the page locked under us.
> > > +             */
> > > +            if (!split_huge_page(page)) {
> > > +                unlock_page(page);
> > > +                put_page(page);
> > > +                /* Re-lookup page cache from current index */
> > > +                goto again;
> > > +            }
> > > +
> > > +            /*
> > > +             * Split fail, clear the page then move to next
> > > +             * index.
> > > +             */
> > > +            clear_highpage(page);
> > > +
> > > +            unlock_page(page);
> > > +            put_page(page);
> > > +        }
> > > +
> > >           index++;
> > >       }
> > >   --
> > > 1.8.3.1