linux-kernel - Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap

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Message-ID: <ZWA9EVlsuHIBi0xb@casper.infradead.org>
Date:   Fri, 24 Nov 2023 06:05:05 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Peng Zhang <zhangpeng362@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, fengwei.yin@...el.com,
        ying.huang@...el.com, aneesh.kumar@...ux.ibm.com,
        shy828301@...il.com, hughd@...gle.com, david@...hat.com,
        wangkefeng.wang@...wei.com, sunnanyong@...wei.com
Subject: Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in
 filemap_fault()

On Wed, Nov 22, 2023 at 10:00:52PM +0800, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@...wei.com>
> 
> The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE)
> in application, which leading to an unexpected performance issue[1].
> 
> This caused by temporarily cleared pte during a read/modify/write update
> of the pte, eg, do_numa_page()/change_pte_range().

What I haven't quite understood yet is why we need to set the pte to
zero on x86 in the specific case of do_numa_page().  I understand that
ppc needs to.

Could someone explain why the _default_ definition of
ptep_modify_prot_start() is not:

+++ b/include/linux/pgtable.h
@@ -1074,7 +1074,7 @@ static inline pte_t ptep_modify_prot_start(struct vm_area_struct *vma,
                                           unsigned long addr,
                                           pte_t *ptep)
 {
-       return __ptep_modify_prot_start(vma, addr, ptep);
+       return *ptep;
 }

 /*