Re: [PATCH] mm/gup: don't check page lru flag before draining it

[David Hildenbrand]

On 05.06.24 03:18, yangge1116 wrote:
> 在 2024/6/4 下午9:47, David Hildenbrand 写道:
> > On 04.06.24 12:48, yangge1116@xxxxxxx wrote:
> > > From: yangge <yangge1116@xxxxxxx>
> > >
> > > If a page is added in pagevec, its ref count increases one, remove
> > > the page from pagevec decreases one. Page migration requires the
> > > page is not referenced by others except page mapping. Before
> > > migrating a page, we should try to drain the page from pagevec in
> > > case the page is in it, however, folio_test_lru() is not sufficient
> > > to tell whether the page is in pagevec or not, if the page is in
> > > pagevec, the migration will fail.
> > >
> > > Remove the condition and drain lru once to ensure the page is not
> > > referenced by pagevec.
> >
> > What you are saying is that we might have a page on which
> > folio_test_lru() succeeds, that was added to one of the cpu_fbatches,
> > correct?
>
> Yes
>
> > Can you describe under which circumstances that happens?
>
> If we call folio_activate() to move a page from inactive LRU list to
> active LRU list, the page is not only in LRU list, but also in one of
> the cpu_fbatches.
>
> void folio_activate(struct folio *folio)
> {
>       if (folio_test_lru(folio) && !folio_test_active(folio) &&
>           !folio_test_unevictable(folio)) {
>           struct folio_batch *fbatch;
>
>           folio_get(folio);
>           //After this, folio is in LRU list, and its ref count have
> increased one.
>
>           local_lock(&cpu_fbatches.lock);
>           fbatch = this_cpu_ptr(&cpu_fbatches.activate);
>           folio_batch_add_and_move(fbatch, folio, folio_activate_fn);
>           local_unlock(&cpu_fbatches.lock);
>       }
> }

Interesting, the !SMP variant does the folio_test_clear_lru().

It would be really helpful if we could reliably identify whether LRU 
batching code has a raised reference on a folio.

We have the same scenario in
* folio_deactivate()
* folio_mark_lazyfree()

In folio_batch_move_lru() we do the folio_test_clear_lru(folio).

No expert on that code, I'm wondering if we could move the 
folio_test_clear_lru() out, such that we can more reliably identify 
whether a folio is on the LRU batch or not.

--
Cheers,

David / dhildenb