linux-kernel - Re: [RFC PATCH 54/61] afs: Wait on PG_fscache before modifying/releasing a page

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Message-ID: <1040571.1588948788@warthog.procyon.org.uk>
Date:   Fri, 08 May 2020 15:39:48 +0100
From:   David Howells <dhowells@...hat.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Jeff Layton <jlayton@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 54/61] afs: Wait on PG_fscache before modifying/releasing a page

David Howells <dhowells@...hat.com> wrote:

> ITER_MAPPING relies on the mapping to maintain the pointers to the pages so
> that it can find them rather than being like ITER_BVEC where there's a
> separate list.
> 
> Truncate removes the pages from the mapping - at which point ITER_MAPPING can
> no longer find them.

It looks like ITER_MAPPING is fine with truncate, provided the invalidation
waits for the iterator to complete first:

	int truncate_inode_page(struct address_space *mapping, struct page *page)
	{
		VM_BUG_ON_PAGE(PageTail(page), page);

		if (page->mapping != mapping)
			return -EIO;

		truncate_cleanup_page(mapping, page);
		delete_from_page_cache(page);
		return 0;
	}

In which case, ->invalidatepage() needs to wait for PG_fscache.

Similarly, it looks like ->releasepage() is fine, provided it waits for
PG_fscache also.

If I have to use ITER_BVEC, what's the advisability of using vmalloc() to
allocate the bio_vec array for a transient op?  Such an array can reference up
to 1MiB on a 64-bit machine with 4KiB non-compound pages if it only allocates
up to a single page.  I'm wondering what the teardown cost is, though, if all
the corresponding PTEs have to be erased from all CPUs.

David