Re: [RFC][PATCH] problem of cont_prepare_write()

[Anton Altaparmakov]

On Mon, 22 Nov 2004, Andrew Morton wrote:
> Anton Altaparmakov <aia21@xxxxxxxxx> wrote:
> >
> > Would it be ok to modify i_size from prepare_write?
> 
> I think so - I seem to recall seeing that done somewhere else...

Great.

> One thing to watch out for is when to bring the page uptodate.  If the 
> page is uptodate then the read() code just won't try to lock it at all.  
> If you increase i_size AND set PG_uptodate too early you could open a 
> window which allows read() to peek at uninitialised data.
> 
> But if you set the page uptodate only when its contents really are sane
> then things should work OK.

Yes, I would never set PG_Uptodate without having sane page contents 
first.

In fact all the metadata writing code in NTFS actually clears PG_Uptodate 
on a locked page because it mangles the page contents, writes them out, 
and then demangles them again, before finally setting PG_Uptodate again 
to protect against read_cache_page() getting hold of pages with i/o in 
flight which for NTFS metadata would cause corruption.

> If you end up doing
> 
> 	memset(page, 0, N);
> 	SetPageUptodate(page);
> 
> then I think you'll need an smb_wmb() in between so the read() code sees the
> above two writes in the correct order.

Thanks.  I always have a flush_dcache_page(page) between the memset() and 
the SetPageUptodate() so I don't need the barrier, right?  Or does the 
flush_dcache_page() not imply ordering?  (I naively thought it did...)

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/
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