Re: [PATCH] mm/rmap: vm_flags including VM_EXEC can exit timely

[zhiguojiang]

在 2023/10/24 23:51, David Hildenbrand 写道:
> On 24.10.23 16:49, Zhiguo Jiang wrote:
> > When pra->vm_flags include VM_EXEC flag and folio is file detected in
> > folio_referenced_one(), the folio referenced traversal process can be
> > exited timely to reduce the detecting folio referenced time.
>
> Can you further elaborate what the logic behind that is?
>
> Why can we stop here if we're dealing with a pagecache folio in an 
> executable VMA?
Functions call flow：folio_referenced() --> rmap_walk() --> 
rmap_walk_ksm()/rmap_walk_anon()/rmap_walk_file() --> rwc->rmap_one() 
--> folio_referenced_one(). And folio_referenced() is called by two 
interfaces: folio_check_references() and shrink_active_list().

1. folio_check_references():
When (referenced_ptes > 0 && (vm_flags & VM_EXEC) && 
folio_is_file_lru(folio)) is detected in folio_check_references(), 
FOLIOREF_ACTIVATE will be returned and the folio will be added to the 
active file lru. So when VM_EXEC is detected in folio_referenced_one(), 
we can stop continuing to detect the reference relationship between this 
folio and other vmas, and exit directly to avoid unnecessary traversal.

2. shrink_active_list():
The shrink_active_list() is the same as the folio_check_references().

> > Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
> > ---
> >   mm/rmap.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >   mode change 100644 => 100755 mm/rmap.c
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 7a27a2b41802..932f3b7e8521
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -884,6 +884,8 @@ static bool folio_referenced_one(struct folio 
> > *folio,
> >       if (referenced) {
> >           pra->referenced++;
> >           pra->vm_flags |= vma->vm_flags & ~VM_LOCKED;
> > +        if ((pra->vm_flags | VM_EXEC) && folio_is_file_lru(folio))
> > +            return false;
> >       }
> >         if (!pra->mapcount)