Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[Lance Yang <ioworker0@gmail.com>]

On Thu, Apr 11, 2024 at 9:48 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> [...]
>
> >>> +
> >>> +             if (!folio_trylock(folio))
> >>> +                     continue;
> >>
> >> This is still wrong. This should all be protected by the "if
> >> (folio_test_swapcache(folio) || folio_test_dirty(folio))" as it was previously
> >> so that you only call folio_trylock() if that condition is true. You are
> >> unconditionally locking here, then unlocking, then relocking below if the
> >> condition is met. Just put everything inside the condition and lock once.
> >
> > I'm not sure if it's safe to call folio_mapcount() without holding the
> > folio lock.
> >
> > As mentioned earlier by David in the v2[1]
> >> What could work for large folios is making sure that #ptes that map the
> >> folio here correspond to the folio_mapcount(). And folio_mapcount()
> >> should be called under folio lock, to avoid racing with swapout/migration.
> >
> > [1] https://lore.kernel.org/all/5cc05529-eb80-410e-bc26-233b0ba0b21f@redhat.com/
>
> But I'm not suggesting that you should call folio_mapcount() without the lock.
> I'm proposing this:
>
>                 if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
>                         if (!folio_trylock(folio))
>                                 continue;
>                         /*
> -                        * If folio is shared with others, we mustn't clear
> -                        * the folio's dirty flag.
> +                        * If we have a large folio at this point, we know it is
> +                        * fully mapped so if its mapcount is the same as its
> +                        * number of pages, it must be exclusive.
>                          */
> -                       if (folio_mapcount(folio) != 1) {
> +                       if (folio_mapcount(folio) != folio_nr_pages(folio)) {
>                                 folio_unlock(folio);
>                                 continue;
>                         }

IIUC, if the folio is clean and not in the swapcache, we still need to
compare the number of batched PTEs against folio_mapcount().

Thanks,
Lance

>
> What am I missing?
>