Re: [PATCH v5 2/2] mm,thp: Add experimental config option RO_EXEC_FILEMAP_HUGE_FAULT_THP

[Michal Hocko]

On Tue 03-09-19 05:22:08, Matthew Wilcox wrote:
> On Tue, Sep 03, 2019 at 02:14:24PM +0200, Michal Hocko wrote:
> > On Mon 02-09-19 03:23:41, William Kucharski wrote:
> > > Add filemap_huge_fault() to attempt to satisfy page
> > > faults on memory-mapped read-only text pages using THP when possible.
> > 
> > This deserves much more description of how the thing is implemented and
> > expected to work. For one thing it is not really clear to me why you
> > need CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP at all. You need a support
> > from the filesystem anyway. So who is going to enable/disable this
> > config?
> 
> There are definitely situations in which enabling this code will crash
> the kernel.  But we want to get filesystems to a point where they can
> start working on their support for large pages.  So our workaround is
> to try to get the core pieces merged under a CONFIG_I_KNOW_WHAT_IM_DOING
> flag and let people play with it.  Then continue to work on the core
> to eliminate those places that are broken.

I am not sure I understand. Each fs has to opt in to the feature
anyway. If it doesn't then there should be no risk of regression, right?
I do not expect any fs would rush an implementation in while not being
sure about the correctness. So how exactly does a config option help
here.
 
> > I cannot really comment on fs specific parts but filemap_huge_fault
> > sounds convoluted so much I cannot wrap my head around it. One thing
> > stand out though. The generic filemap_huge_fault depends on ->readpage
> > doing the right thing which sounds quite questionable to me. If nothing
> > else  I would expect ->readpages to do the job.
> 
> Ah, that's because you're not a filesystem person ;-)  ->readpages is
> really ->readahead.  It's a crappy interface and should be completely
> redesigned.

OK, the interface looked like the right fit for this purpose. Thanks for
clarifying.
-- 
Michal Hocko
SUSE Labs