Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering tlb flush when folios get unmapped

[Huang, Ying]

David Hildenbrand <david@xxxxxxxxxx> writes:

> On 03.06.24 19:01, Matthew Wilcox wrote:
>> On Mon, Jun 03, 2024 at 09:37:46AM -0700, Dave Hansen wrote:
>>> Yeah, we'd need some equivalent of a PTE marker, but for the page cache.
>>>   Presumably some xa_value() that means a reader has to go do a
>>> luf_flush() before going any farther.
>> I can allocate one for that.  We've got something like 1000
>> currently
>> unused values which can't be mistaken for anything else.
>
> I'm curious when to set that, though.
>
> While migrating/reclaiming, when unmapping the folio from the page
> tables, the folio is still valid in the page cache. So at the point in
> time of unmapping from one process, we cannot simply replace the folio
> in the page cache by some other value -- I think.
>
> Maybe it's all easier than I think.

IIUC, we need to held folio lock before replacing the folio in the page
cache.  In page_cache_delete(), folio_test_locked() is checked.  And, we
will lock the folio before writing to it via write syscall.  So, it's
safe to defer TLB flushing until we unlock the folio.

--
Best Regards,
Huang, Ying