linux-kernel - Re: [PATCH 2/2] fs: hugetlbfs: Fix logic to skip allocation on hit in page cache

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Message-ID: <20230503030528.GC3873@monkey>
Date:   Tue, 2 May 2023 20:05:28 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Ackerley Tng <ackerleytng@...gle.com>, willy@...radead.org,
        sidhartha.kumar@...cle.com
Cc:     akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        muchun.song@...ux.dev, jhubbard@...dia.com, vannapurve@...gle.com,
        erdemaktas@...gle.com
Subject: Re: [PATCH 2/2] fs: hugetlbfs: Fix logic to skip allocation on hit
 in page cache

On 05/02/23 23:56, Ackerley Tng wrote:
> When fallocate() is called twice on the same offset in the file, the
> second fallocate() should succeed.
> 
> page_cache_next_miss() always advances index before returning, so even
> on a page cache hit, the check would set present to false.

Thank you Ackerley for finding this!

When I read the description of page_cache_next_miss(), I assumed

	present = page_cache_next_miss(mapping, index, 1) != index;

would tell us if there was a page at index in the cache.

However, when looking closer at the code it does not check for a page
at index, but rather starts looking at index+1.  Perhaps that is why
it is named next?

Matthew, I think the use of the above statement was your suggestion.
And you know the xarray code better than anyone.  I just want to make
sure page_cache_next_miss is operating as designed/expected.  If so,
then the changes suggested here make sense.

In addition, the same code is in hugetlbfs_pagecache_present and will
have this same issue.
-- 
Mike Kravetz

> 
> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
> ---
>  fs/hugetlbfs/inode.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index ecfdfb2529a3..f640cff1bbce 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -821,7 +821,6 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>  		 */
>  		struct folio *folio;
>  		unsigned long addr;
> -		bool present;
>  
>  		cond_resched();
>  
> @@ -845,10 +844,7 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>  		mutex_lock(&hugetlb_fault_mutex_table[hash]);
>  
>  		/* See if already present in mapping to avoid alloc/free */
> -		rcu_read_lock();
> -		present = page_cache_next_miss(mapping, index, 1) != index;
> -		rcu_read_unlock();
> -		if (present) {
> +		if (filemap_has_folio(mapping, index)) {
>  			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>  			hugetlb_drop_vma_policy(&pseudo_vma);
>  			continue;
> -- 
> 2.40.1.495.gc816e09b53d-goog
>