Re: [PATCH v4 3/3] mm: Batch-zap large anonymous folio PTE mappings

From: David Hildenbrand
Date: Thu Aug 03 2023 - 09:58:10 EST

On 27.07.23 19:22, Yu Zhao wrote:
On Thu, Jul 27, 2023 at 8:18 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:

This allows batching the rmap removal with folio_remove_rmap_range(),
which means we avoid spuriously adding a partially unmapped folio to the
deferred split queue in the common case, which reduces split queue lock

Previously each page was removed from the rmap individually with
page_remove_rmap(). If the first page belonged to a large folio, this
would cause page_remove_rmap() to conclude that the folio was now
partially mapped and add the folio to the deferred split queue. But
subsequent calls would cause the folio to become fully unmapped, meaning
there is no value to adding it to the split queue.

A complicating factor is that for platforms where MMU_GATHER_NO_GATHER
is enabled (e.g. s390), __tlb_remove_page() drops a reference to the
page. This means that the folio reference count could drop to zero while
still in use (i.e. before folio_remove_rmap_range() is called). This
does not happen on other platforms because the actual page freeing is

Solve this by appropriately getting/putting the folio to guarrantee it
does not get freed early. Given the need to get/put the folio in the
batch path, we stick to the non-batched path if the folio is not large.
While the batched path is functionally correct for a folio with 1 page,
it is unlikely to be as efficient as the existing non-batched path in
this case.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

This ad hoc patch looks unacceptable to me: we can't afford to keep
adding special cases.

I vote for completely converting zap_pte_range() to use
folio_remove_rmap_range(), and that includes tlb_flush_rmap_batch()
and other types of large folios, not just anon. Otherwise I'll leave
it to Matthew and David.

The !anon case with tlb_delay_rmap() really hurts my brain (again).

We're already special-casing on !anon ... so splitting anon and !anon also doesn't sound completely off to me.

But yes, we should find ways to just avoid any special casing here, or at least minimize them.

(The bigger problem I have with this patch, as raised in my replies, is that it messes up the order in which we adjust mapcount+refcount, and I am *really* not a friend of that :) )


David / dhildenb