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From: David Howells <dhowells@redhat.com>
To: Trond Myklebust <trondmy@hammerspace.com>
Cc: dhowells@redhat.com, dwysocha@redhat.com,
	anna.schumaker@netapp.com, Matthew Wilcox <willy@infradead.org>,
	linux-cachefs@redhat.com, linux-nfs@vger.kernel.org,
	linux-mm@kvack.org
Subject: Can the GFP flags to releasepage() be trusted? -- was Re: [PATCH v2 3/8] nfs: Move to using the alternate fallback fscache I/O API
Date: Fri, 01 Oct 2021 15:51:56 +0100	[thread overview]
Message-ID: <81120.1633099916@warthog.procyon.org.uk> (raw)
In-Reply-To: <97eb17f51c8fd9a89f10d9dd0bf35f1075f6b236.camel@hammerspace.com>

Trond Myklebust <trondmy@hammerspace.com> wrote:

> > > @@ -432,7 +432,12 @@ static int nfs_release_page(struct page *page, gfp_t gfp)
> > >  	/* If PagePrivate() is set, then the page is not freeable */
> > >  	if (PagePrivate(page))
> > >  		return 0;
> > > -	return nfs_fscache_release_page(page, gfp);
> > > +	if (PageFsCache(page)) {
> > > +		if (!(gfp & __GFP_DIRECT_RECLAIM) || !(gfp & __GFP_FS))
> > > +			return false;
> > > +		wait_on_page_fscache(page);
> > > +	}
> > > +	return true;
> > >  }
> 
> I've found this generally not to be safe. The VM calls ->release_page()
> from a variety of contexts, and often fails to report it correctly in
> the gfp flags. That's particularly true of the stuff in mm/vmscan.c.
> This is why we have the check above that vetos page removal upon
> PagePrivate() being set.

[Adding Willy and the mm crew to the cc list]

I wonder if that matters in this case.  In the worst case, we'll wait for the
page to cease being DMA'd - but we won't return true if it is.

But if vmscan is generating the wrong VM flags, we should look at fixing that.

David


  parent reply	other threads:[~2021-10-01 14:52 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-17 15:04 [RFC PATCH v2 0/8] fscache: Replace and remove old I/O API v2 David Howells
2021-09-17 15:04 ` [PATCH v2 1/8] fscache: Generalise the ->begin_read_operation method David Howells
2021-09-17 15:04 ` [PATCH v2 2/8] fscache: Implement a fallback I/O interface to replace the old API David Howells
2021-09-17 15:04 ` [PATCH v2 3/8] nfs: Move to using the alternate fallback fscache I/O API David Howells
2021-09-29 14:44   ` David Wysochanski
2021-10-01 14:42   ` David Howells
     [not found]   ` <CALF+zO=165sRYRaxPpDS7DaQCpTe-YOa4FamSoMy5FV2uuG5Yg@mail.gmail.com>
2021-10-01 14:29     ` Trond Myklebust
2021-10-01 14:51     ` David Howells [this message]
2021-10-01 15:04       ` Can the GFP flags to releasepage() be trusted? -- was " Trond Myklebust
2021-10-01 19:42         ` Matthew Wilcox
2021-10-05 13:15       ` David Howells
2021-09-17 15:04 ` [PATCH v2 4/8] 9p: (untested) Convert to using the netfs helper lib to do reads and caching David Howells
2021-09-17 15:05 ` [PATCH v2 5/8] cifs: (untested) Move to using the alternate fallback fscache I/O API David Howells
2021-09-17 15:05 ` [PATCH v2 6/8] fscache: Remove the old " David Howells
2021-09-17 15:05 ` [PATCH v2 7/8] fscache: Remove stats that are no longer used David Howells
2021-09-17 15:05 ` [PATCH v2 8/8] fscache: Update the documentation to reflect I/O API changes David Howells

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