Messages in this thread | | | Date | Thu, 28 May 2009 22:50:21 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3 |
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On Thu, May 28, 2009 at 09:45:20PM +0800, Andi Kleen wrote: > On Thu, May 28, 2009 at 02:08:54PM +0200, Nick Piggin wrote:
[snip]
> > > > BTW. I don't know if you are checking for PG_writeback often enough? > > You can't remove a PG_writeback page from pagecache. The normal > > pattern is lock_page(page); wait_on_page_writeback(page); which I > > So pages can be in writeback without being locked? I still > wasn't able to find such a case (in fact unless I'm misreading > the code badly the writeback bit is only used by NFS and a few > obscure cases)
Yes the writeback page is typically not locked. Only read IO requires to be exclusive. Read IO is in fact page *writer*, while writeback IO is page *reader* :-)
The writeback bit is _widely_ used. test_set_page_writeback() is directly used by NFS/AFS etc. But its main user is in fact set_page_writeback(), which is called in 26 places.
> > think would be safest > > Okay. I'll just add it after the page lock. > > > (then you never have to bother with the writeback bit again) > > Until Fengguang does something fancy with it.
Yes I'm going to do it without wait_on_page_writeback().
The reason truncate_inode_pages_range() has to wait on writeback page is to ensure data integrity. Otherwise if there comes two events: truncate page A at offset X populate page B at offset X If A and B are all writeback pages, then B can hit disk first and then be overwritten by A. Which corrupts the data at offset X from user's POV.
But for hwpoison, there are no such worries. If A is poisoned, we do our best to isolate it as well as intercepting its IO. If the interception fails, it will trigger another machine check before hitting the disk.
After all, poisoned A means the data at offset X is already corrupted. It doesn't matter if there comes another B page.
Thanks, Fengguang
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