Re: [PATCH 0/2] overcommit: introduce mem-lock-onfault

[Peter Xu]

On Tue, Dec 10, 2024 at 08:01:08PM +0300, Daniil Tatianin wrote:
> I mentioned my use case in the cover letter. Basically we want to protect
> QEMU's pages from being migrated and compacted by kcompactd, which it
> accomplishes by modifying live page tables and spamming the process with TLB
> invalidate IPIs while it does that, which kills guest performance for the
> duration of the compaction operation.

Ah right, I read it initially but just now when I scanned the cover letter
I missed that.  My fault.

> 
> Memory locking allows to protect a process from kcompactd page compaction
> and more importantly, migration (that is taking a PTE and replacing it with
> one, which is closer in memory to reduce fragmentation). (As long as
> /proc/sys/vm/compact_unevictable_allowed is 0)
> 
> For this use case we don't mind page faults as they take more or less
> constant time, which we can also avoid if we wanted by preallocating guest
> memory. We do, however, want PTEs to be untouched by kcompactd, which
> MCL_ONFAULT accomplishes just fine without the extra memory overhead that
> comes from various anonymous mappings getting write-faulted with the
> currently available mem-lock=on option.
> 
> In our case we use KVM of course, TCG was just an experiment where I noticed
> anonymous memory
> jump way too much.
> 
> I don't think it's feasible in our case to look for the origin of every
> anonymous mapping that grew compared to the no mem-lock case (which there's
> about ~30 with default Q35 + KVM, without any extra devices), and try to
> optimize it to map anonymous memory less eagerly.

Would it be better then to use mem-lock=on|off|onfault?  So turns it into a
string to avoid the "exclusiveness" needed (meanwhile having two separate
knobs for relevant things looks odd too).

Thanks,

-- 
Peter Xu