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

[Yang Shi]

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