Re: [PATCH v2 0/3] Enable THP for text section of non-shmem files

[Song Liu <songliubraving@fb.com>]

> On Jun 18, 2019, at 2:12 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Fri, 14 Jun 2019 11:22:01 -0700 Song Liu <songliubraving@fb.com> wrote:
> 
>> This set follows up discussion at LSF/MM 2019. The motivation is to put
>> text section of an application in THP, and thus reduces iTLB miss rate and
>> improves performance. Both Facebook and Oracle showed strong interests to
>> this feature.
>> 
>> To make reviews easier, this set aims a mininal valid product. Current
>> version of the work does not have any changes to file system specific
>> code. This comes with some limitations (discussed later).
>> 
>> This set enables an application to "hugify" its text section by simply
>> running something like:
>> 
>>          madvise(0x600000, 0x80000, MADV_HUGEPAGE);
>> 
>> Before this call, the /proc/<pid>/maps looks like:
>> 
>>    00400000-074d0000 r-xp 00000000 00:27 2006927     app
>> 
>> After this call, part of the text section is split out and mapped to THP:
>> 
>>    00400000-00425000 r-xp 00000000 00:27 2006927     app
>>    00600000-00e00000 r-xp 00200000 00:27 2006927     app   <<< on THP
>>    00e00000-074d0000 r-xp 00a00000 00:27 2006927     app
>> 
>> Limitations:
>> 
>> 1. This only works for text section (vma with VM_DENYWRITE).
>> 2. Once the application put its own pages in THP, the file is read only.
>>   open(file, O_WRITE) will fail with -ETXTBSY. To modify/update the file,
>>   it must be removed first.
> 
> Removed?  Even if the original mmap/madvise has gone away?  hm.
> 
> I'm wondering if this limitation can be abused in some fashion: mmap a
> file to which you have read permissions, run madvise(MADV_HUGEPAGE) and
> thus prevent the file's owner from being able to modify the file?  Or
> something like that.  What are the issues and protections here?
> 

I found a better solution to this limitation. Please refer to changes
in v3 (especially 6/6). 

Thanks,
Song