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    SubjectRe: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3
    On Tue, Jun 02, 2009 at 08:57:13PM +0800, Nick Piggin wrote:
    > On Tue, Jun 02, 2009 at 02:47:57PM +0200, Andi Kleen wrote:
    > > On Tue, Jun 02, 2009 at 02:00:42PM +0200, Nick Piggin wrote:
    > > > not a big deal and just avoids duplicating code. I attached an
    > > > (untested) patch.
    > >
    > > Thanks. But the function in the patch is not doing the same what
    > > the me_pagecache_clean/dirty are doing. For once there is no error
    > > checking, as in the second try_to_release_page()
    > >
    > > Then it doesn't do all the IO error and missing mapping handling.
    >
    > Obviously I don't mean just use that single call for the entire
    > handler. You can set the EIO bit or whatever you like. The
    > "error handling" you have there also seems strange. You could
    > retain it, but the page is assured to be removed from pagecache.

    You mean this?

    if (page_has_private(p) && !try_to_release_page(p, GFP_NOIO))
    return FAILED;

    If page->private cannot be removed, that means some fs may start IO on it, so
    we return FAILED.

    > > The page_mapped() check is useless because the pages are not
    > > mapped here etc.
    >
    > That's OK, it is a core part of the protocol to prevent
    > truncated pages from being mapped, so I like it to be in
    > that function.

    Right.

    > (you are also doing extraneous page_mapped tests in your handler,
    > so surely your concern isn't from the perspective of this
    > error handler code)

    That's because the initial try_to_unmap() may fail and page still remain
    mapped, and remove_from_page_cache() assumes !page_mapped().

    > > We could probably call truncate_complete_page(), but then
    > > we would also need to duplicate most of the checking outside
    > > the function anyways and there wouldn't be any possibility
    > > to share the clean/dirty variants. If you insist I can
    > > do it, but I think it would be significantly worse code
    > > than before and I'm reluctant to do that.
    >
    > I can write you the patch for that too if you like.

    I have already posted one on truncate_complete_page(). Not the way you want it?

    > > I don't also really see what the big deal is of just
    > > calling these few functions directly. After all we're not
    > > truncating here and they're all already called from other files.
    > >
    > > > > > No, it seems rather insane to do something like this here that no other
    > > > > > code in the mm ever does.
    > > > >
    > > > > Just because the rest of the VM doesn't do it doesn't mean it might make sense.
    > > >
    > > > It is going to be possible to do it somehow surely, but it is insane
    > > > to try to add such constraints to the VM to close a few small windows
    > >
    > > We don't know currently if they are small. If they are small I would
    > > agree with you, but that needs numbers. That said fancy writeback handling
    > > is currently not on my agenda.
    >
    > Yes, writeback pages are very limited, a tiny number at any time and
    > the faction gets relatively smaller as total RAM size gets larger.

    Yes they are less interesting for now.

    > > > if you already have other large ones.
    > >
    > > That's unclear too.
    >
    > You can't do much about most kernel pages, and dirty metadata pages
    > are both going to cause big problems. User pagetable pages. Lots of
    > stuff.

    Yes, that's a network of pointers that's hard to break away with.

    Thanks,
    Fengguang


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