linux-kernel - Re: [patch 1/2] mm: dont clear PG_uptodate in invalidate_complete

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Message-Id: <E1KBY4N-0007YD-5n@pomaz-ex.szeredi.hu>
Date:	Wed, 25 Jun 2008 18:42:07 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	torvalds@...ux-foundation.org
CC:	miklos@...redi.hu, jens.axboe@...cle.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, akpm@...ux-foundation.org, hugh@...itas.com,
	nickpiggin@...oo.com.au
Subject: Re: [patch 1/2] mm: dont clear PG_uptodate in
 invalidate_complete_page2()

> > 
> > We discussed this yesterday.  My conclusion was (which I still think
> > is true) that it can't be fixed in page_cache_pipe_buf_confirm(),
> > because due to current practice of not setting PG_error for I/O errors
> > for read, it is impossible to distinguish between a never-been-uptodate
> > page and a was-uptodate-before-invalidation page.
> 
> Umm. The regular read does this quite well. If something isn't up-to-date, 
> it tries a synchronous read. Once.

Exactly.  And if page_cache_pipe_buf_confirm() could do a synchronous
re-read of the page, that would work.  But it can't, because it only
has the page and not the file.

> > And it's not just an nfsd issue.  Userspace might also expect that if
> > a zero count is returned, that means it went beyond EOF, and not that
> > it should retry the splice, maybe it has better luck this time.
> 
> You're totally ignoring the real issue - user space that uses splice() 
> *knows* that it uses splice(). It's a private mmap(). 
> 
> NFSD, on the other hand, is supposed to act as NFSD. I think that 
> currently it assumes that nobody else modifies the files, which is 
> reasonable, but breaks with FUSE.

Not so.  Why couldn't someone modify an ext3 file, while nfsd is
holding the page?  Is that wrong?  I don't know, but it's not fuse
specific.

> But do you see? That's a NFSD/FUSE issue, not a splice one!

No, I think you are wrong.

Miklos
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