Messages in this thread | | | Date | Wed, 13 Jun 2012 14:29:49 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 2/2] msync: start async writeout when MS_ASYNC |
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On Thu, 31 May 2012 22:43:55 +0200 Paolo Bonzini <pbonzini@redhat.com> wrote:
> msync.c says that applications had better use fsync() or fadvise(FADV_DONTNEED) > instead of MS_ASYNC. Both advices are really bad: > > * fsync() can be a replacement for MS_SYNC, not for MS_ASYNC; > > * fadvise(FADV_DONTNEED) invalidates the pages completely, which will make > later accesses expensive. > > Having the possibility to schedule a writeback immediately is an advantage > for the applications. They can do the same thing that fadvise does, > but without the invalidation part. The implementation is also similar > to fadvise, but with tag-and-write enabled. > > One example is if you are implementing a persistent dirty bitmap. > Whenever you set bits to 1 you need to synchronize it with MS_SYNC, so > that dirtiness is reported properly after a host crash. If you have set > any bits to 0, getting them to disk is not needed for correctness, but > it is still desirable to save some work after a host crash. You could > simply use MS_SYNC in a separate thread, but MS_ASYNC provides exactly > the desired semantics and is easily done in the kernel. > > If the application does not want to start I/O, it can simply call msync > with flags equal to MS_INVALIDATE. This one remains a no-op, as it should > be on a reasonable implementation.
Means that people will find that their msync(MS_ASYNC) call will newly start IO. This may well be undesirable for some.
Also, it hardwires into the kernel behaviour which userspace itself could have initiated, with sync_file_range(). ie: reduced flexibility.
Perhaps we can update the msync.c code comments to direct people to sync_file_range()?
One wonders how msync() works with nonlinear mappings. I guess "badly". I think this was all discussed when we merged remap_file_pages() (what a mistake that was) and we decided "too hard".
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