Re: [RFC PATCH 3/4] netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator

[David Howells <dhowells@redhat.com>]

I updated the commit message to stop using pinning in a general sense:

    netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator
    
    Add a function to extract the pages from a user-space supplied iterator
    (UBUF- or IOVEC-type) into a BVEC-type iterator, retaining the pages by
    getting a ref on them (WRITE) or pinning them (READ) as we go.
    
    This is useful in three situations:
    
     (1) A userspace thread may have a sibling that unmaps or remaps the
         process's VM during the operation, changing the assignment of the
         pages and potentially causing an error.  Retaining the pages keeps
         some pages around, even if this occurs; futher, we find out at the
         point of extraction if EFAULT is going to be incurred.
    
     (2) Pages might get swapped out/discarded if not retained, so we want to
         retain them to avoid the reload causing a deadlock due to a DIO
         from/to an mmapped region on the same file.
    
     (3) The iterator may get passed to sendmsg() by the filesystem.  If a
         fault occurs, we may get a short write to a TCP stream that's then
         tricky to recover from.
    
    We don't deal with other types of iterator here, leaving it to other
    mechanisms to retain the pages (eg. PG_locked, PG_writeback and the pipe
    lock).

David