Re: [PATCH] multifd: Copy pages before compressing them with zlib

[Juan Quintela]

Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> zlib_send_prepare() compresses pages of a running VM. zlib does not
> make any thread-safety guarantees with respect to changing deflate()
> input concurrently with deflate() [1].
>
> One can observe problems due to this with the IBM zEnterprise Data
> Compression accelerator capable zlib [2]. When the hardware
> acceleration is enabled, migration/multifd/tcp/plain/zlib test fails
> intermittently [3] due to sliding window corruption. The accelerator's
> architecture explicitly discourages concurrent accesses [4]:
>
>     Page 26-57, "Other Conditions":
>
>     As observed by this CPU, other CPUs, and channel
>     programs, references to the parameter block, first,
>     second, and third operands may be multiple-access
>     references, accesses to these storage locations are
>     not necessarily block-concurrent, and the sequence
>     of these accesses or references is undefined.
>
> Mark Adler pointed out that vanilla zlib performs double fetches under
> certain circumstances as well [5], therefore we need to copy data
> before passing it to deflate().
>
> [1] https://zlib.net/manual.html
> [2] https://github.com/madler/zlib/pull/410
> [3] https://lists.nongnu.org/archive/html/qemu-devel/2022-03/msg03988.html
> [4] http://publibfp.dhe.ibm.com/epubs/pdf/a227832c.pdf
> [5] https://gitlab.com/qemu-project/qemu/-/issues/1099
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

And now I wonder if we need this for zstd.

Once told that, compression (not multifd one) has always operated the
other way, sniff.