linux-kernel - Re: [RFC v2 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

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Date:   Tue, 24 Apr 2018 07:08:36 -0600
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        hch@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH] mm: shmem: make stat.st_blksize return huge page
 size if THP is on



On 4/24/18 6:43 AM, Michal Hocko wrote:
> On Mon 23-04-18 21:41:50, Yang Shi wrote:
>>
>> On 4/23/18 9:04 AM, Michal Hocko wrote:
>>> On Sun 22-04-18 21:28:59, Yang Shi wrote:
>>>> On 4/22/18 6:47 PM, Michal Hocko wrote:
>>> [...]
>>>>> will be used on the first aligned address even when the initial/last
>>>>> portion of the mapping is not THP aligned.
>>>> No, my test shows it is not. And, transhuge_vma_suitable() does check the
>>>> virtual address alignment. If it is not huge page size aligned, it will not
>>>> set PMD for huge page.
>>> It's been quite some time since I've looked at that code but I think you
>>> are wrong. It just doesn't make sense to make the THP decision on the
>>> VMA alignment much. Kirill, can you clarify please?
>> Thanks a lot Michal and Kirill to elaborate how tmpfs THP make pmd map.
>>
>> I did a quick test, THP will be PMD mapped as long as :
>> * hint address is huge page aligned if MAP_FIXED
>> Or
>> * offset is huge page aligned
>> And
>> * The size is big enough (>= huge page size)
>>
>> This test does verify what Kirill said. And, I dig into a little further
>> qemu code and did strace, qemu does try to mmap the file to non huge page
>> aligned address with MAP_FIXED.
> Does it make sense to contact Qemu developers and probably fix this?

Yes, I think so. We can submit a bug report to QEMU.

>