Re: [PATCH v3] mm/mprotect: try avoiding write faults for exclusive anonymous pages when changing protection

[David Hildenbrand <david@redhat.com>]

On 15.06.22 17:25, Peter Xu wrote:
> On Tue, Jun 14, 2022 at 11:36:29AM +0200, David Hildenbrand wrote:
>> Similar to our MM_CP_DIRTY_ACCT handling for shared, writable mappings, we
>> can try mapping anonymous pages in a private writable mapping writable if
>> they are exclusive, the PTE is already dirty, and no special handling
>> applies. Mapping the anonymous page writable is essentially the same thing
>> the write fault handler would do in this case.
>>
>> Special handling is required for uffd-wp and softdirty tracking, so take
>> care of that properly. Also, leave PROT_NONE handling alone for now;
>> in the future, we could similarly extend the logic in do_numa_page() or
>> use pte_mk_savedwrite() here.
>>
>> While this improves mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE)
>> performance, it should also be a valuable optimization for uffd-wp, when
>> un-protecting.
>>
>> This has been previously suggested by Peter Collingbourne in [1],
>> relevant in the context of the Scudo memory allocator, before we had
>> PageAnonExclusive.
>>
>> This commit doesn't add the same handling for PMDs (i.e., anonymous THP,
>> anonymous hugetlb); benchmark results from Andrea indicate that there
>> are minor performance gains, so it's might still be valuable to streamline
>> that logic for all anonymous pages in the future.
>>
>> As we now also set MM_CP_DIRTY_ACCT for private mappings, let's rename
>> it to MM_CP_TRY_CHANGE_WRITABLE, to make it clearer what's actually
>> happening.
> 
> I'm personally not sure why DIRTY_ACCT cannot be applied to private
> mappings; it sounds not only for shared but a common thing.  I also don't

TBH, I think the name is just absolutely unclear in that context.

> know whether "change writable" could be misread too anyway. Say, we're
> never changing RO->RW mappings with this flag, but only try to unprotect
> the page proactively when proper, from that POV Nadav's suggestion seems
> slightly better on using "unprotect".

write unprotection is a change from RO->RW, so I don't immediately see
the difference.

Anyhow, I don't like the sounding of TRY_WRITE_UNPROTECT.

I made it match the function name that I had:

MM_CP_TRY_CHANGE_WRITABLE
-> !pte_write()?
 -> can_change_pte_writable() ?
  ->pte_mkwrite()

Maybe MM_CP_TRY_MAKE_WRITABLE / MM_CP_TRY_MAKE_PTE_WRITABLE is clearer?

Open for suggestions because I'm apparently not the bast at naming
things either.

> 
> No strong opinion, the patch looks correct to me, and thanks for providing
> the new test results,
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> 

Thanks Peter!

-- 
Thanks,

David / dhildenb