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:   Mon, 23 Apr 2018 10:19:27 -0600
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>, kirill.shutemov@...ux.intel.com
Cc:     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 4/23/18 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?

In the test, QEMU is trying to mmap a file (16GB in my configuration) + 
a guard page. If the page size is 4KB, there not any pages are mapped by 
PMD, but if the page size is 2MB (huge page aligned) we can see a lot 
pages are mapped by PMD. The test result is showed in the commit log.

So, if your assumption is right, there must be something wrong in THP code.

>
> Please note that I have no objections to actually export the huge page
> size as the max block size but your changelog just doesn't make any
> sense to me.

Thanks,
Yang