linux-kernel - Re: [PATCH v2 2/2] mm/readahead: don't decrease mmap

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Date: Mon, 25 Mar 2024 16:30:46 +0100
From: Jan Kara <jack@...e.cz>
To: Liu Shixin <liushixin2@...wei.com>
Cc: Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v2 2/2] mm/readahead: don't decrease mmap_miss when folio
 has workingset flags

On Fri 22-03-24 17:35:55, Liu Shixin wrote:
> If there are too many folios that are recently evicted in a file, then
> they will probably continue to be evicted. In such situation, there is
> no positive effect to read-ahead this file since it is only a waste of IO.
> 
> The mmap_miss is increased in do_sync_mmap_readahead() and decreased in
> both do_async_mmap_readahead() and filemap_map_pages(). In order to skip
> read-ahead in above scenario, the mmap_miss have to increased exceed
> MMAP_LOTSAMISS. This can be done by stop decreased mmap_miss when folio
> has workingset flags. The async path is not to care because in above
> scenario, it's hard to run into the async path.
> 
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>
..
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 8df4797c5287..753771310127 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3439,7 +3439,8 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>  		if (PageHWPoison(page + count))
>  			goto skip;
>  
> -		(*mmap_miss)++;
> +		if (!folio_test_workingset(folio))
> +			(*mmap_miss)++;

Hum, so this means we consider this a 'hit' if the page is completely new
in the page cache or evicted long time ago. OK, makes sense. It would be
nice to add a comment in this direction to explain the condition. Frankly
the whole mmap_miss accounting is broken as I've outlined in my patch
series. But I guess this works as a fixup for your immediate problem and
we can make mmap_miss accounting sensible later. So for now feel free to
add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR