linux-kernel - Re: [PATCH 2/4] filemap: kill page_cache_read usage in filemap

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Date:   Fri, 7 Dec 2018 10:57:50 +0100
From:   Jan Kara <jack@...e.cz>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     kernel-team@...com, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, tj@...nel.org, david@...morbit.com,
        akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, riel@...hat.com, jack@...e.cz
Subject: Re: [PATCH 2/4] filemap: kill page_cache_read usage in filemap_fault

On Fri 30-11-18 14:58:10, Josef Bacik wrote:
> If we do not have a page at filemap_fault time we'll do this weird
> forced page_cache_read thing to populate the page, and then drop it
> again and loop around and find it.  This makes for 2 ways we can read a
> page in filemap_fault, and it's not really needed.  Instead add a
> FGP_FOR_MMAP flag so that pagecache_get_page() will return a unlocked
> page that's in pagecache.  Then use the normal page locking and readpage
> logic already in filemap_fault.  This simplifies the no page in page
> cache case significantly.
> 
> Signed-off-by: Josef Bacik <josef@...icpanda.com>

Thanks for the patch. I like the simplification but I think it could be
even improved... see below.

> @@ -2449,9 +2426,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>  		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
>  		ret = VM_FAULT_MAJOR;
>  retry_find:
> -		page = find_get_page(mapping, offset);
> +		page = pagecache_get_page(mapping, offset,
> +					  FGP_CREAT|FGP_FOR_MMAP,
> +					  vmf->gfp_mask);
>  		if (!page)
> -			goto no_cached_page;
> +			return vmf_error(-ENOMEM);

So why don't you just do:

		page = pagecache_get_page(mapping, offset,
					  FGP_CREAT | FGP_LOCK, vmf->gfp_mask);
		if (!page)
			return vmf_error(-ENOMEM);
		goto check_uptodate;

where check_uptodate would be a label before 'PageUptodate' check?

Then you don't have to introduce new flag for pagecache_get_page() and you
also don't have to unlock and then lock again the page... And you can still
delete all the code you've deleted.

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