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

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Date:   Wed, 3 May 2023 17:14:09 -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 20:05, Mike Kravetz wrote:
> 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.

I took a closer look at the code today.

page_cache_next_miss has a 'special case' for index 0.  The function
description says:

 * Return: The index of the gap if found, otherwise an index outside the
 * range specified (in which case 'return - index >= max_scan' will be true).
 * In the rare case of index wrap-around, 0 will be returned.

And, the loop in the routine does:

	while (max_scan--) {
		void *entry = xas_next(&xas);
		if (!entry || xa_is_value(entry))
			break;
		if (xas.xa_index == 0)
			break;
	}

At first glance, I thought xas_next always went to the next entry but
now see that is not the case here because this is a new state with
xa_node = XAS_RESTART.  So, xas_next is effectively a xas_load.

This means in the case were index == 0,

	page_cache_next_miss(mapping, index, 1)

will ALWAYS return zero even if a page is present.

I need to look at the xarray code and this rare index wrap-around case
to see if we can somehow modify that check for xas.xa_index == 0 in
page_cache_next_miss.
-- 
Mike Kravetz