Re: Warning on memory offline (and possible in usual migration?)

[Nick Piggin <npiggin@suse.de>]

On Wed, Apr 16, 2008 at 11:36:42AM -0700, Andrew Morton wrote:
> On Wed, 16 Apr 2008 20:00:36 +0900
> > With tons of printk, I think I found when it happens.
> > 
> > Assume I use ia64/PAGE_SIZE=16k and ext3's blocksize=4k.
> > A page has 4 buffer_heads.
> > 
> > Assume that a page is not Uptodate before issuing write_begin()
> > 
> > At the end of writing to ext3, the kernel reaches here.
> > ==
> > static int __block_commit_write(struct inode *inode, struct page *page,
> >                 unsigned from, unsigned to)
> > {
> >     int patrial=0;
> > 
> >     if (!All_buffers_to_this_page_is_uptodate)
> > 	partial = 1
> >     if (!partial)
> >         SetPageUptodate(page)
> > }
> > ==
> > To set a page as Uptodate, all buffers must be uptodate.
> > 
> > But *all* buffers to this page is not necessary to be uptodate, here. 
> > Then, the page can be not-up-to-date after commit-write.
> > 
> > At page offlining, all buffers on the page seems to be marked as Uptodate
> > (by printk) but the page itself isn't. This seems strange.
> > 
> > But I don't found who set Uptodate to the buffers. 
> > And why page isn't up-to-date while all buffers are marked as up-to-date.
> 
> That would imply that someone brought a buffer uptodate and didn't mark the
> page uptodate.  That can happen if a read reads the buffer from disk or
> memsets all of it.  Or if a write memsets all of it, or does
> copy_from_user() into all of it.
> 
> > still chasing.
> 
> umm..
> 
> If you had some code which does
> 
> 	pread(fd, buf, 1, 0);
> 	pread(fd, buf, 1, 4096);
> 	pread(fd, buf, 1, 8192);
> 	pread(fd, buf, 1, 12288);
> 
> then I'd expect that each read would read a single buffer so we end up with
> four uptodate buffers, but nobody brings the entire page uptodate.

The generic read path AFAIK doesn't do partial buffers.

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