linux-kernel - Re: [PATCH v1 2/7] mm: make zap_pte_range() handle full within-PMD range

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Message-ID: <5242273a-e5c8-44f3-93ba-488e64af78c3@bytedance.com>
Date: Fri, 18 Oct 2024 10:23:35 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>
Cc: david@...hat.com, hughd@...gle.com, willy@...radead.org, mgorman@...e.de,
 muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
 zokeefe@...gle.com, rientjes@...gle.com, peterx@...hat.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v1 2/7] mm: make zap_pte_range() handle full within-PMD
 range



On 2024/10/18 02:06, Jann Horn wrote:
> On Thu, Oct 17, 2024 at 11:48 AM Qi Zheng <zhengqi.arch@...edance.com> wrote:
>> In preparation for reclaiming empty PTE pages, this commit first makes
>> zap_pte_range() to handle the full within-PMD range, so that we can more
>> easily detect and free PTE pages in this function in subsequent commits.
> 
> I think your patch causes some unintended difference in behavior:
> 
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>>   mm/memory.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index caa6ed0a7fe5b..fd57c0f49fce2 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1602,6 +1602,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>          swp_entry_t entry;
>>          int nr;
>>
>> +retry:
> 
> This "retry" label is below the line "bool force_flush = false,
> force_break = false;", so I think after force_break is set once and
> you go through the retry path, every subsequent present PTE will again
> bail out and retry. I think that doesn't lead to anything bad, but it
> seems unintended.

Right, thanks for catching this! Will set force_flush and force_break to
false under "retry" label in v2.

Thanks!