On Fri, 30 Jul 2021, Yang Shi wrote:
> On Fri, Jul 30, 2021 at 12:25 AM Hugh Dickins <hughd@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
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
