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

[Lance Yang]

On Thu, Apr 11, 2024 at 10:39 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 11/04/2024 15:07, Lance Yang wrote:
> > On Thu, Apr 11, 2024 at 9:48 PM Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxxxxx/
> >>
> >> 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().
>
> Why? That's not how the old code worked. In fact the comment says that the
> reason for the exclusive check is to avoid marking a dirty *folio* as clean if
> shared; that would be bad because we could throw away data that others relied
> upon. It's perfectly safe to clear the dirty flag from the *pte* even if it is
> shared; the ptes are private to the process so that won't affect sharers.
>
> You should just follow the pattern already estabilished by the original code.
> The only difference is that because the folio is now (potentially) large, you
> have to change the way to detect exclusivity.

Thanks a lot for your patience and help!

My bad for the oversight and mistake :(
I'll take another look at the original code and make adjustments following the
established pattern.

Thanks,
Lance

>
> >
> > Thanks,
> > Lance
> >
> >>
> >> What am I missing?
> >>
>