Re: [patch 5/8] mm: write_cache_pages integrity fix

[Nick Piggin <npiggin@suse.de>]

On Thu, Oct 09, 2008 at 10:39:23AM -0400, Chris Mason wrote:
> On Thu, 2008-10-09 at 16:21 +0200, Nick Piggin wrote:
> > On Thu, Oct 09, 2008 at 10:12:55AM -0400, Chris Mason wrote:
> > > On Thu, 2008-10-09 at 15:55 +0200, Nick Piggin wrote:
> > > > On Thu, Oct 09, 2008 at 09:35:58AM -0400, Chris Mason wrote:
> > > > > On Thu, 2008-10-09 at 15:27 +0200, Nick Piggin wrote:
> > > > > 
> > > > > I don't think do_sync_mapping_range is broken as is.  It simply splits
> > > > > the operations into different parts.  The caller can request that we
> > > > > wait for pending IO first.
> > > > 
> > > > It is. Not because of it's whacky API, but because it uses WB_SYNC_NONE. 
> > > > 
> > > > 
> > > > > WB_SYNC_NONE none just means don't wait for IO in flight, and there are
> > > > > valid uses for it that will slow down if you switch them all to
> > > > > WB_SYNC_ALL.
> > > > 
> > > > To write_cache_pages it means that, but further down the chain (eg.
> > > > block_write_full_page) it also means not to wait on other stuff.
> > > > 
> > > > It has broadly meant "don't worry about data integirty" for a long time
> > > > AFAIKS.
> > > 
> > > Sadly it has broadly meant different things to different people ;)
> > > You're right, block_write_full_page is broken.
> > 
> > Well, I really just think it is do_sync_mapping_range that is broken.
> > Because __sync_single_inode treats WB_SYNC_NONE as a general "nowait",
> > so does __writeback_single_inode. Weakest semantics define the API :)
> 
> Unfortunately these things are using the flag differently
> __sync_single_inode and __writeback_single_inode do different things
> with the flag than people that end up directly calling the writepages
> methods.

Sure, but it's the "nowait" semantics that they want, right? And *they*
eventually call into writepages. So they want similar semantics from
writepages, presumably. 

The comment in WB_SYNC_NONE definition kind of suggests it meant don't
wait for anything when it was written...


> At the write_cache_pages level, WB_SYNC_NONE should only change the
> waiting for IO in flight.

Aside from do_sync_mapping_range, what are other reasons to enforce
the same thing all up and down the writeout stack? If there are good
reasons, let's add WB_SYNC_WRITEBACK?