Re: [PATCH 2/4] mm: compaction: convert to folio_isolate_movable()

[Vishal Moola <vishal.moola@gmail.com>]

On Wed, Aug 28, 2024 at 08:36:03PM +0800, Kefeng Wang wrote:
> 
> 
> On 2024/8/28 18:04, Baolin Wang wrote:
> > Hi Kefeng,
> > 
> > On 2024/8/26 12:01, Kefeng Wang wrote:
> > > The tail page will always fail to isolate for non-lru-movable and
> > > LRU page in isolate_migratepages_block(), so move PageTail check
> > > ahead, then convert to use folio_isolate_movable() helper and more
> > > folios.
> > > 
> > > Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> > > ---
> > >   mm/compaction.c | 32 +++++++++++++++++---------------
> > >   1 file changed, 17 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index a2b16b08cbbf..aa2e8bb9fa58 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -1074,39 +1074,41 @@ isolate_migratepages_block(struct
> > > compact_control *cc, unsigned long low_pfn,
> > >               }
> > >           }
> > > +        if (PageTail(page))
> > > +            goto isolate_fail;
> > > +
> > > +        folio = page_folio(page);
> > 
> > I wonder if this is stable. Before page_folio(), it does not hold a
> > reference on the page, so seems we should re-check the folio still
> > contains this page after gaining a reference on the folio?
> 
> Oh, you are right, so two way to avoid this,
> 
> 1) re-check 'page_folio(page)  == folio', but this need change a little
> more.
> 
> 2)  directly use folio_get_nontail_page(page) here, and folio_put in the
> following path, this will try to get for any pages, but it should be
> accepted ?
> 
> I'd prefer 2) but any other suggestion?

I think option 2 makes sense, and simply use folio_put() on success and
goto isolate_fail_put on failure instead of isolate_fail.

With option 2, it might make sense to have folio_isolate_movable()
expect to be passed a folio with elevated refcount. Then it could be
treated similarly to its sister function folio_isolate_lru() and simply
use folio_get() for its reference.