linux-ext4 - Re: Best way to pin a page in ext4?

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Date:	Tue, 16 Sep 2014 21:25:42 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-mm <linux-mm@...ck.org>, linux-ext4@...r.kernel.org
Subject: Re: Best way to pin a page in ext4?

On Tue, Sep 16, 2014 at 05:07:18PM -0700, Hugh Dickins wrote:
> You might (I'm not certain) be able to get away with extending the
> use of mlock_vma_page() and munlock_vma_page() in this (admittedly
> attractive) way, up until someone mmap's that range (and mlocks
> then munlocks it? again, I'm not certain if that's necessary).
> Then the PageMlocked flag is liable to be cleared, because the
> page will not be found in any mlock'ed vma; and the page can
> then be reclaimed behind your back (statistics gone wrong too?
> again I'm not sure).
> 
> Now, I expect it's unlikely (impossible?) for anyone to mmap your
> bitmap pages while they're being used as filesystem metadata (rather
> than mere blockdev pages).  But you can see why we would prefer not
> to export those functions.

Yes, it's impossible for anyone to mmap the pages from
EXT4_SB(sb)->s_buddy_cache inode, because it's not exposed in any way
to userspace.  But I can see why you wouldn't want it to be used
almost anywhere else.

> For now I agree with Andreas, just grab an extra refcount; but you're
> right that leaving these pages on evictable LRUs is regrettable,
> and can be inefficient under reclaim.

OK, fair enough, that seems simpler all around.

Cheers,

							- Ted
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