Re: [PATCH v5 6/7] mm: Optimize mprotect() by PTE batching

[Will Deacon <will@kernel.org>]

On Wed, Aug 06, 2025 at 10:08:33AM +0200, David Hildenbrand wrote:
> On 18.07.25 11:02, Dev Jain wrote:
> > Use folio_pte_batch to batch process a large folio. Note that, PTE
> > batching here will save a few function calls, and this strategy in certain
> > cases (not this one) batches atomic operations in general, so we have
> > a performance win for all arches. This patch paves the way for patch 7
> > which will help us elide the TLBI per contig block on arm64.
> > 
> > The correctness of this patch lies on the correctness of setting the
> > new ptes based upon information only from the first pte of the batch
> > (which may also have accumulated a/d bits via modify_prot_start_ptes()).
> > 
> > Observe that the flag combination we pass to mprotect_folio_pte_batch()
> > guarantees that the batch is uniform w.r.t the soft-dirty bit and the
> > writable bit. Therefore, the only bits which may differ are the a/d bits.
> > So we only need to worry about code which is concerned about the a/d bits
> > of the PTEs.
> > 
> > Setting extra a/d bits on the new ptes where previously they were not set,
> > is fine - setting access bit when it was not set is not an incorrectness
> > problem but will only possibly delay the reclaim of the page mapped by
> > the pte (which is in fact intended because the kernel just operated on this
> > region via mprotect()!). Setting dirty bit when it was not set is again
> > not an incorrectness problem but will only possibly force an unnecessary
> > writeback.
> > 
> > So now we need to reason whether something can go wrong via
> > can_change_pte_writable(). The pte_protnone, pte_needs_soft_dirty_wp,
> > and userfaultfd_pte_wp cases are solved due to uniformity in the
> > corresponding bits guaranteed by the flag combination. The ptes all
> > belong to the same VMA (since callers guarantee that [start, end) will
> > lie within the VMA) therefore the conditional based on the VMA is also
> > safe to batch around.
> > 
> > Since the dirty bit on the PTE really is just an indication that the folio
> > got written to - even if the PTE is not actually dirty but one of the PTEs
> > in the batch is, the wp-fault optimization can be made. Therefore, it is
> > safe to batch around pte_dirty() in can_change_shared_pte_writable()
> > (in fact this is better since without batching, it may happen that
> > some ptes aren't changed to writable just because they are not dirty,
> > even though the other ptes mapping the same large folio are dirty).
> > 
> > To batch around the PageAnonExclusive case, we must check the corresponding
> > condition for every single page. Therefore, from the large folio batch,
> > we process sub batches of ptes mapping pages with the same
> > PageAnonExclusive condition, and process that sub batch, then determine
> > and process the next sub batch, and so on. Note that this does not cause
> > any extra overhead; if suppose the size of the folio batch is 512, then
> > the sub batch processing in total will take 512 iterations, which is the
> > same as what we would have done before.
> > 
> > For pte_needs_flush():
> > 
> > ppc does not care about the a/d bits.
> > 
> > For x86, PAGE_SAVED_DIRTY is ignored. We will flush only when a/d bits
> > get cleared; since we can only have extra a/d bits due to batching,
> > we will only have an extra flush, not a case where we elide a flush due
> > to batching when we shouldn't have.
> > 
> > Signed-off-by: Dev Jain <dev.jain@arm.com>
> 
> 
> I wanted to review this, but looks like it's already upstream and I suspect
> it's buggy (see the upstream report I cc'ed you on)

Please excuse my laziness, but do you have a link to the report? I've
been looking at some oddities on arm64 coming back from some of the CI
systems and was heading in the direction of a recent mm regression
judging by the first-known-bad-build in linux-next.

https://lore.kernel.org/r/CA+G9fYumD2MGjECCv0wx2V_96_FKNtFQpT63qVNrrCmomoPYVQ@mail.gmail.com

Will