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Re: [PATCH v1 0/3] util/oslib-posix: Support MADV_POPULATE_WRITE for os_

From: Daniel P . Berrangé
Subject: Re: [PATCH v1 0/3] util/oslib-posix: Support MADV_POPULATE_WRITE for os_mem_prealloc()
Date: Tue, 20 Jul 2021 15:45:26 +0100
User-agent: Mutt/2.0.7 (2021-05-04)

On Wed, Jul 14, 2021 at 01:23:03PM +0200, David Hildenbrand wrote:
> #1 adds support for MADV_POPULATE_WRITE, #2 cleans up the code to avoid
> global variables and prepare for concurrency and #3 makes os_mem_prealloc()
> safe to be called from multiple threads concurrently.
> Details regarding MADV_POPULATE_WRITE can be found in introducing upstream
> Linux commit 4ca9b3859dac ("mm/madvise: introduce
> MADV_POPULATE_(READ|WRITE) to prefault page tables") and in the latest man
> page patch [1].

Looking at that commit message, I see your caveat about POPULATE_WRITE
used together with shared file mappings, causing an undesirable glut
of dirty pages that needs to be flushed back to the underlying storage.

Is this something we need to be concerned with for the hostmem-file.c
implementation ? While it is mostly used to point to files on tmpfs
or hugetlbfs, I think users do something point it to a plain file
on a normal filesystem.  So will we need to optimize to use the
fallocate+POPULATE_READ combination at some point ?

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