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

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Date: Tue, 04 Jun 2024 16:16:49 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,  Dave Hansen
 <dave.hansen@...el.com>,  Byungchul Park <byungchul@...com>,  Byungchul
 Park <lkml.byungchul.park@...il.com>,  linux-kernel@...r.kernel.org,
  linux-mm@...ck.org,  kernel_team@...ynix.com,  akpm@...ux-foundation.org,
  vernhao@...cent.com,  mgorman@...hsingularity.net,  hughd@...gle.com,
  peterz@...radead.org,  luto@...nel.org,  tglx@...utronix.de,
  mingo@...hat.com,  bp@...en8.de,  dave.hansen@...ux.intel.com,
  rjgolo@...il.com
Subject: Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering
 tlb flush when folios get unmapped

David Hildenbrand <david@...hat.com> 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