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

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Date:   Tue, 1 Nov 2022 18:31:26 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 3/5] userfualtfd: Replace lru_cache functions with
 folio_add functions

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!

> 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
> 
>