Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to allocate more pages per call

[Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>]

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...