[lustre-devel] [PATCH] mm: Avoid returning VM_FAULT_RETRY from ->page_mkwrite handlers

Jan Kara jack at suse.cz
Fri Feb 3 08:10:05 PST 2017

On Fri 03-02-17 07:53:26, Matthew Wilcox wrote:
> On Fri, Feb 03, 2017 at 04:46:40PM +0100, Jan Kara wrote:
> > On Fri 03-02-17 07:13:59, Matthew Wilcox wrote:
> > > On Fri, Feb 03, 2017 at 04:07:29PM +0100, Jan Kara wrote:
> > > > Some ->page_mkwrite handlers may return VM_FAULT_RETRY as its return
> > > > code (GFS2 or Lustre can definitely do this). However VM_FAULT_RETRY
> > > > from ->page_mkwrite is completely unhandled by the mm code and results
> > > > in locking and writeably mapping the page which definitely is not what
> > > > the caller wanted. Fix Lustre and block_page_mkwrite_ret() used by other
> > > > filesystems (notably GFS2) to return VM_FAULT_NOPAGE instead which
> > > > results in bailing out from the fault code, the CPU then retries the
> > > > access, and we fault again effectively doing what the handler wanted.
> > > 
> > > Reading this commit message makes me wonder if this is the best fix.
> > > It would seem logical that if I want the fault to be retried that I should
> > > return VM_FAULT_RETRY, not VM_FAULT_NOPAGE.  Why don't we have the MM
> > > treat VM_FAULT_RETRY the same way that it treats VM_FAULT_NOPAGE and give
> > > driver / filesystem writers one fewer way to shoot themselves in the foot?
> > 
> > VM_FAULT_RETRY is special, it may be used only if FAULT_FLAG_ALLOW_RETRY
> > was set in page fault flags and it means - we have dropped mmap_sem, we
> > loaded page needed to satisfy the fault and now we need to try again (have
> > a look at __lock_page_or_retry()). I have my reservations about this
> > interface but it works...
> Oh, I understand what it's *supposed* to be used for ;-)  It's just
> a bit of an attractive nuisance.  Maybe renaming it to something like
> VM_FAULT_PAGE_RETRY would stop people from thinking that it meant "retry
> the fault".  And we could #define VM_FAULT_RETRY VM_FAULT_NOPAGE so that
> people who want to retry the fault in a normal way could use a return
> value that sounds like it does what they want instead of a return value
> that is supposed to be used to indicate that we put a PFN into the
> page table?

So a better name for VM_FAULT_RETRY and disentangling VM_FAULT_NOPAGE so
that it is not misused for retrying the fault would be nice. But it is a
much larger endeavor (I actually had a look into this some time ago) than
this simple bugfix...

Jan Kara <jack at suse.com>

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