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

[yangge1116]

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);
    }
}