linux-kernel - Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with folio

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Message-ID: <Y2K+y7wnhC4vbnP2@x1n>
Date:   Wed, 2 Nov 2022 15:02:35 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
        Hugh Dickins <hughd@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with
 folio_add functions

On Tue, Nov 01, 2022 at 06:31:26PM +0000, Matthew Wilcox wrote:
> On Tue, Nov 01, 2022 at 10:53:24AM -0700, Vishal Moola (Oracle) wrote:
> > Replaces lru_cache_add() and lru_cache_add_inactive_or_unevictable()
> > with folio_add_lru() and folio_add_lru_vma(). This is in preparation for
> > the removal of lru_cache_add().
> 
> Ummmmm.  Reviewing this patch reveals a bug (not introduced by your
> patch).  Look:
> 
> mfill_atomic_install_pte:
>         bool page_in_cache = page->mapping;
> 
> mcontinue_atomic_pte:
>         ret = shmem_get_folio(inode, pgoff, &folio, SGP_NOALLOC);
> ...
>         page = folio_file_page(folio, pgoff);
>         ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
>                                        page, false, wp_copy);
> 
> That says pretty plainly that mfill_atomic_install_pte() can be passed
> a tail page from shmem, and if it is ...
> 
>         if (page_in_cache) {
> ...
>         } else {
>                 page_add_new_anon_rmap(page, dst_vma, dst_addr);
>                 lru_cache_add_inactive_or_unevictable(page, dst_vma);
>         }
> 
> it'll get put on the rmap as an anon page!

Hmm yeah.. thanks Matthew!

Does the patch attached look reasonable to you?

Copying Axel too.

> 
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
> > ---
> >  mm/userfaultfd.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index e24e8a47ce8a..2560973b00d8 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -66,6 +66,7 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> >  	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> >  	bool page_in_cache = page->mapping;
> >  	spinlock_t *ptl;
> > +	struct folio *folio;
> >  	struct inode *inode;
> >  	pgoff_t offset, max_off;
> >  
> > @@ -113,14 +114,15 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> >  	if (!pte_none_mostly(*dst_pte))
> >  		goto out_unlock;
> >  
> > +	folio = page_folio(page);
> >  	if (page_in_cache) {
> >  		/* Usually, cache pages are already added to LRU */
> >  		if (newly_allocated)
> > -			lru_cache_add(page);
> > +			folio_add_lru(folio);
> >  		page_add_file_rmap(page, dst_vma, false);
> >  	} else {
> >  		page_add_new_anon_rmap(page, dst_vma, dst_addr);
> > -		lru_cache_add_inactive_or_unevictable(page, dst_vma);
> > +		folio_add_lru_vma(folio, dst_vma);
> >  	}
> >  
> >  	/*
> > -- 
> > 2.38.1
> > 
> > 
> 

-- 
Peter Xu

View attachment "0001-mm-shmem-Use-page_mapping-to-detect-page-cache-for-u.patch" of type "text/plain" (1899 bytes)