On Thu 02-02-17 18:28:02, Al Viro wrote:
> On Thu, Feb 02, 2017 at 03:48:17PM +0100, Jan Kara wrote:
>
> > > * ->page_mkwrite() instances sometimes return VM_FAULT_RETRY; AFAICS,
> > > it's only (ab)used there as 'not zero, but doesn't contain any error bits';
> > > VM_FAULT_RETRY from that source does *not* reach handle_mm_fault() callers,
> > > right?
> >
> > I can see only Lustre doing it and IMHO it is abuse. VM_FAULT_RETRY is used
> > for mmap_sem latency reduction when paging in pages and so not everybody
> > handles it. If a handler wants to simply retry the fault, returning
> > VM_FAULT_NOPAGE is a more common way to do that...
>
> /* Convert errno to return value from ->page_mkwrite() call */
> static inline int block_page_mkwrite_return(int err)
> {
> if (err == 0)
> return VM_FAULT_LOCKED;
> if (err == -EFAULT)
> return VM_FAULT_NOPAGE;
> if (err == -ENOMEM)
> return VM_FAULT_OOM;
> if (err == -EAGAIN)
> return VM_FAULT_RETRY;
> /* -ENOSPC, -EDQUOT, -EIO ... */
> return VM_FAULT_SIGBUS;
> }
>
> and a bunch of ->page_mkwrite() instances using that. However, the only
> callers of ->page_mkwrite() are wp_page_shared()->do_page_mkwrite() and
> do_shared_fault()->do_page_mkwrite(). do_page_mkwrite() treates
> VM_FAULT_RETRY as "lock page and return VM_FAULT_RETRY|VM_FAULT_LOCKED".
> Both callers do the same check -
> if (unlikely(!tmp || (tmp &
> (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
> and the return value if that predicate is false. FWIW, use of VM_FAULT_RETRY
> comes from your patch back in 2011 and AFAICS the same analysis used to
> apply back then, except for the open-coded method calls where we use
> do_page_mkwrite() these days...
Yeah, back then I was not aware of VM_FAULT_RETRY limitations and your
analysis above just shows that its handling from do_page_mkwrite() is
simply broken (or better non-existent). Actually that VM_FAULT_RETRY
return was added by fs freeze handling patch. The freeze handling was
later changed but that change to block_page_mkwrite_return() remained.
I'll send a patch to remove it. Thanks for spotting this.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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