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

[Hugh Dickins <hughd@google.com>]

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?

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