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Re: [Qemu-devel] [RFC 2/2] block/file-posix: verify page cache is not us
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [RFC 2/2] block/file-posix: verify page cache is not used |
Date: |
Fri, 20 Apr 2018 08:25:13 +0200 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
Am 20.04.2018 um 05:02 hat Stefan Hajnoczi geschrieben:
> On Thu, Apr 19, 2018 at 10:05:47AM +0100, Dr. David Alan Gilbert wrote:
> > * Stefan Hajnoczi (address@hidden) wrote:
> > > This commit is for debugging only. Do not merge it.
> > >
> > > mincore(2) checks whether pages are resident. Use it to verify that
> > > page cache has been dropped.
> > >
> > > You can trigger a verification failure by mmapping the image file from
> > > another process and loading a byte from a page so that it becomes
> > > resident. bdrv_co_invalidate_cache() will fail while the process is
> > > alive.
> >
> > It doesn't seem a bad diagnostic to keep in (with a switch to activate)
> > for when we're faced with some weird corruption on some weird storage
> > system.
>
> Okay. It's very slow to mmap an entire image file and query mincore(2)
> so it needs to be off by default.
Also, having it enabled breaks localhost migration at least on tmpfs
(which was what I tried out first).
I wonder if the kernel would add some way to query whether the "advice"
was actually acted upon if we asked. Either with a new function that
returns an error if not everything is dropped (basically
.bdrv_invalidate_cache on the kernel level), or a function that just
queries if any page is allocated (or maybe the address of the first
allocated page in a given range) without having to use mincore() and
iterating over all the pages in userspace.
Kevin
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[Qemu-devel] [RFC 2/2] block/file-posix: verify page cache is not used, Stefan Hajnoczi, 2018/04/19
Re: [Qemu-devel] [RFC 0/2] block/file-posix: allow -drive cache.direct=off live migration, Eric Blake, 2018/04/19