Messages in this thread Patch in this message | | | Date | Wed, 10 Apr 2024 10:10:37 +0200 | Subject | Re: [PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again | From | David Hildenbrand <> |
| |
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
| |