Re: aio_fsync

[Andrew Morton <akpm@osdl.org>]

Matthew Wilcox <willy@debian.org> wrote:
>
> i'm updating the Documentation/filesystem/Locking document.
> i notice that sys_fsync() takes the i_sem before calling ->fsync, but
> io_submit_one() does not take i_sem before calling ->aio_fsync.
> 
> is this intentional?  do we not need to protect against concurrent
> writers as sys_fsync does?

I don't think anything implements aio_fsync().

But yes, the main reason for taking i_sem there is to prevent new writes
from coming in and hopelessly livelocking the fsync.

Unless the fsync() is implemented via a radix-tree walk.  That is not
livelockable: just do a sync_page_range() across the pages from 0 to
i_size.  This can potentially blow some amounts of CPU if there are a lot
of pages in cache but only a handful are dirty.

fsync() is lame.  If we go ahead and implement this I would suggest that we
allow userspace to pass in a start/length which represents a subsection of
the file.  That's easy to do and is a ton more useful.  And all the needed
bits and pieces already exist in the O_SYNC handling in -mm: it's just a
matter of wiring it up.


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