Re: [PATCH] mm, thp: Relax the VM_DENYWRITE constraint on file-backed THPs

[Song Liu]

> On Mar 23, 2021, at 10:13 AM, Collin Fijalkovich <cfijalkovich@xxxxxxxxxx> wrote:
> 
> Question: when we use this on shared library, the library is still
> writable. When the
> shared library is opened for write, these pages will refault in as 4kB
> pages, right? 
> 
> That's correct, while a file is opened for write it will refault into 4kB pages and block use of THPs. Once all writers complete (i_writecount <=0), the file can fault into THPs again and khugepaged can collapse existing page ranges provided that it can successfully allocate new huge pages.

Will it be a problem if a slow writer (say a slow scp) writes to the 
shared library while the shared library is in use? 

Thanks,
Song

> 
> From,
> Collin 
> 
> On Mon, Mar 22, 2021 at 4:55 PM Song Liu <song@xxxxxxxxxx> wrote:
> On Mon, Mar 22, 2021 at 3:00 PM Collin Fijalkovich
> <cfijalkovich@xxxxxxxxxx> wrote:
> >
> > Transparent huge pages are supported for read-only non-shmem filesystems,
> > but are only used for vmas with VM_DENYWRITE. This condition ensures that
> > file THPs are protected from writes while an application is running
> > (ETXTBSY).  Any existing file THPs are then dropped from the page cache
> > when a file is opened for write in do_dentry_open(). Since sys_mmap
> > ignores MAP_DENYWRITE, this constrains the use of file THPs to vmas
> > produced by execve().
> >
> > Systems that make heavy use of shared libraries (e.g. Android) are unable
> > to apply VM_DENYWRITE through the dynamic linker, preventing them from
> > benefiting from the resultant reduced contention on the TLB.
> >
> > This patch reduces the constraint on file THPs allowing use with any
> > executable mapping from a file not opened for write (see
> > inode_is_open_for_write()). It also introduces additional conditions to
> > ensure that files opened for write will never be backed by file THPs.
> 
> Thanks for working on this. We could also use this in many data center
> workloads.
> 
> Question: when we use this on shared library, the library is still
> writable. When the
> shared library is opened for write, these pages will refault in as 4kB
> pages, right?
> 
> Thanks,
> Song