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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 18 Jun 2019 21:48:16 +0000 Song Liu <songliubraving@fb.com> wrote:

> > 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?
> 
> In this case, the owner need to make a copy of the file, and then remove 
> and update the original file. 
> 
> In this version, we want either split huge page on writes, or fail the 
> write when we cannot split. However, the huge page information is only 
> available at page level, and on the write path, page level information 
> is not available until write_begin(). So it is hard to stop writes at 
> earlier stage. Therefore, in this version, we leverage i_mmap_writable, 
> which is at address_space level. So it is easier to stop writes to the 
> file. 
> 
> This is a temporary behavior. And it is gated by the config. So I guess
> it is OK. It works well for our use cases though. Once we have better 
> write support, we can remove the limitation. 
> 
> If this is too weird, I am also open to suggestions. 

Well, it's more than weird?  This permits user A to deny service to
user B?  User A can, maliciously or accidentally, prevent user B from
modifying a file which user B has permission to modify?  Such as, umm,
/etc/hosts?