linux-kernel - Re: [PATCH RFC] hugetlbfs 'noautofill' mount option

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Date:   Sun, 7 May 2017 22:57:19 -0700
From:   Prakash Sangappa <prakash.sangappa@...cle.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH RFC] hugetlbfs 'noautofill' mount option



On 5/3/17 12:02 PM, Prakash Sangappa wrote:
> On 5/2/17 4:43 PM, Dave Hansen wrote:
>
>> Ideally, it would be something that is *not* specifically for hugetlbfs.
>>   MADV_NOAUTOFILL, for instance, could be defined to SIGSEGV whenever
>> memory is touched that was not populated with MADV_WILLNEED, mlock(), 
>> etc...
>
> If this is a generic advice type, necessary support will have to be 
> implemented
> in various filesystems which can support this.
>
> The proposed behavior for 'noautofill' was to not fill holes in 
> files(like sparse files).
> In the page fault path, mm would not know if the mmapped address on which
> the fault occurred, is over a hole in the file or just that the page 
> is not available
> in the page cache. The underlying filesystem would be called and it 
> determines
> if it is a hole and that is where it would fail and not fill the hole, 
> if this support is added.
> Normally, filesystem which support sparse files(holes in file) 
> automatically fill the hole
> when accessed. Then there is the issue of file system block size and 
> page size. If the
> block sizes are smaller then page size, it could mean the noautofill 
> would only work
> if the hole size is equal to  or a multiple of, page size?
>
> In case of hugetlbfs it is much straight forward. Since this 
> filesystem is not like a normal
> filesystems and and the file sizes are multiple of huge pages. The 
> hole will be a multiple
> of the huge page size. For this reason then should the advise be 
> specific to hugetlbfs?
>
>


Any further comments? I think introducing a general madvise option or a 
mmap flag applicable to all filesystems, may not be required. The 
'noautofill' behavior would be specifically useful in hugetlbfs filesystem.

So, if it is specific to hugetlbfs, will the mount option be ok? 
Otherwise adding a madvise / mmap option specific to hugetlbfs, be 
preferred?