Re: [PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again

David Hildenbrand
2024-04-10T08:10:43Z

On 09.04.24 23:42, Matthew Wilcox wrote:
> On Tue, Apr 09, 2024 at 09:22:44PM +0200, David Hildenbrand wrote:
>> Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type()
>> pages") made it impossible to detect mapcount underflows by treating
>> any negative raw mapcount value as a mapcount of 0.
>
> Yes, but I don't think this is the right place to check for underflow.
> We should be checking for that on modification, not on read.

While I don't disagree (and we'd check more instances that way, for example
deferred rmap removal), that requires a bit more churn and figuring out of
if losing some information we would have printed in print_bad_pte() is worth
that change.

> I think
> it's more important for page_mapcount() to be fast than a debugging aid.

I really don't think page_mapcount() is a good use of time for
micro-optimizations, but let's investigate:

A big hunk of code in page_mapcount() seems to be the compound handling.
The code before that (reading mapcount, checking for the condition,
conditionally setting it to 0), would generate right now:

177: 8b 42 30 mov 0x30(%rdx),%eax
17a: b9 00 00 00 00 mov $0x0,%ecx
17f: 83 c0 01 add $0x1,%eax
182: 0f 48 c1 cmovs %ecx,%eax

My variant is longer:

17b: 8b 4a 30 mov 0x30(%rdx),%ecx
17e: 81 f9 7f ff ff ff cmp $0xffffff7f,%ecx
184: 8d 41 01 lea 0x1(%rcx),%eax
187: b9 00 00 00 00 mov $0x0,%ecx
18c: 0f 4e c1 cmovle %ecx,%eax
18f: 48 8b 0a mov (%rdx),%rcx

The compiler does not seem to do the smart thing, which would
be rearranging the code to effectively be:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef34cf54c14f..7392596882ae 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1232,7 +1232,7 @@ static inline int page_mapcount(struct page *page)
int mapcount = atomic_read(&page->_mapcount) + 1;

/* Handle page_has_type() pages */
- if (mapcount < 0)
+ if (mapcount < PAGE_MAPCOUNT_RESERVE + 1)
mapcount = 0;
if (unlikely(PageCompound(page)))
mapcount += folio_entire_mapcount(page_folio(page));

Which would result in:

177: 8b 42 30 mov 0x30(%rdx),%eax
17a: 31 c9 xor %ecx,%ecx
17c: 83 c0 01 add $0x1,%eax
17f: 83 f8 80 cmp $0xffffff80,%eax
182: 0f 4e c1 cmovle %ecx,%eax

Same code length, one more instruction. No jumps.

I can switch to the above (essentially inlining
page_type_has_type()) for now and look into different sanity checks --
and extending the documentation around page_mapcount() behavior for
underflows -- separately.

... unless you insist that we really have to change that immediately.

Thanks!

--
Cheers,

David / dhildenb