Re: [PATCH 01/16] huge tmpfs: fix fallocate(vanilla) advance over huge pages

[Yang Shi <shy828301@gmail.com>]

On Sat, Jul 31, 2021 at 8:38 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Fri, 30 Jul 2021, Yang Shi wrote:
> > On Fri, Jul 30, 2021 at 12:25 AM Hugh Dickins <hughd@google.com> wrote:
> > >
> > > shmem_fallocate() goes to a lot of trouble to leave its newly allocated
> > > pages !Uptodate, partly to identify and undo them on failure, partly to
> > > leave the overhead of clearing them until later.  But the huge page case
> > > did not skip to the end of the extent, walked through the tail pages one
> > > by one, and appeared to work just fine: but in doing so, cleared and
> > > Uptodated the huge page, so there was no way to undo it on failure.
> > >
> > > Now advance immediately to the end of the huge extent, with a comment on
> > > why this is more than just an optimization.  But although this speeds up
> > > huge tmpfs fallocation, it does leave the clearing until first use, and
> > > some users may have come to appreciate slow fallocate but fast first use:
> > > if they complain, then we can consider adding a pass to clear at the end.
> > >
> > > Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> > > Signed-off-by: Hugh Dickins <hughd@google.com>
> >
> > Reviewed-by: Yang Shi <shy828301@gmail.com>
>
> Many thanks for reviewing so many of these.
>
> >
> > A nit below:
> >
> > > ---
> > >  mm/shmem.c | 19 ++++++++++++++++---
> > >  1 file changed, 16 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 70d9ce294bb4..0cd5c9156457 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -2736,7 +2736,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> > >         inode->i_private = &shmem_falloc;
> > >         spin_unlock(&inode->i_lock);
> > >
> > > -       for (index = start; index < end; index++) {
> > > +       for (index = start; index < end; ) {
> > >                 struct page *page;
> > >
> > >                 /*
> > > @@ -2759,13 +2759,26 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> > >                         goto undone;
> > >                 }
> > >
> > > +               index++;
> > > +               /*
> > > +                * Here is a more important optimization than it appears:
> > > +                * a second SGP_FALLOC on the same huge page will clear it,
> > > +                * making it PageUptodate and un-undoable if we fail later.
> > > +                */
> > > +               if (PageTransCompound(page)) {
> > > +                       index = round_up(index, HPAGE_PMD_NR);
> > > +                       /* Beware 32-bit wraparound */
> > > +                       if (!index)
> > > +                               index--;
> > > +               }
> > > +
> > >                 /*
> > >                  * Inform shmem_writepage() how far we have reached.
> > >                  * No need for lock or barrier: we have the page lock.
> > >                  */
> > > -               shmem_falloc.next++;
> > >                 if (!PageUptodate(page))
> > > -                       shmem_falloc.nr_falloced++;
> > > +                       shmem_falloc.nr_falloced += index - shmem_falloc.next;
> > > +               shmem_falloc.next = index;
> >
> > This also fixed the wrong accounting of nr_falloced, so it should be
> > able to avoid returning -ENOMEM prematurely IIUC. Is it worth
> > mentioning in the commit log?
>
> It took me a long time to see your point there: ah yes, because it made
> the whole huge page Uptodate when it reached the first tail, there would
> have been only one nr_falloced++ for the whole of the huge page: well
> spotted, thanks, I hadn't realized that.
>
> Though I'm not so sure about your premature -ENOMEM: because once it has
> made the huge page Uptodate, the other end (shmem_writepage()) will not
> be incrementing nr_unswapped at all: so -ENOMEM would have been deferred
> rather than premature, wouldn't it?

Ah, ok, I didn't pay too much attention to how nr_unswapped is
incremented. Just thought nr_falloced will be incremented by 512
rather than 1, so it is more unlikely to return -ENOMEM.

>
> Add a comment on this in the commit log: yes, I guess so, but I haven't
> worked out what to write yet.
>
> Hugh
>
> >
> > >
> > >                 /*
> > >                  * If !PageUptodate, leave it that way so that freeable pages
> > > --
> > > 2.26.2