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

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Message-ID: <20180424124312.GZ17484@dhcp22.suse.cz>
Date:   Tue, 24 Apr 2018 06:43:12 -0600
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
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 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?

-- 
Michal Hocko
SUSE Labs