Messages in this thread | | | Subject | Re: [PATCH] aoe: adjust ref of head for compound page tails | From | Ed Cashin <> | Date | Wed, 7 Aug 2013 19:54:45 -0400 |
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On Aug 7, 2013, at 7:35 PM, Andrew Morton wrote:
> On Wed, 7 Aug 2013 19:27:40 -0400 Ed Cashin <ecashin@coraid.com> wrote: > >> On Aug 7, 2013, at 5:26 PM, Andrew Morton wrote: >> >>> On Wed, 7 Aug 2013 17:21:32 -0400 Ed Cashin <ecashin@coraid.com> wrote: >> ... >>>> Sorry. I tried to cover that but maybe should have separated it more clearly from the running text. >>>> >>>> The patch changes the current behavior encountered with direct I/O and aoe: >>>> >>>> aoe can BUG when direct I/O used >>>> >>>> ... to a new behavior: >>>> >>>> aoe does not BUG when direct I/O is used >>>> >>>> And the user will never know that instead aoe is manipulating the _count of the head page, even though that's what's happening with this patch applied. >>>> >>>> (The user-visible effects that motivated the older patch that added the BUG to aoe when page tails are used for block I/O was a VM_BUG_ON that occurred when the aoe driver manipulated the _count of a compound page tail.) >>> >>> OK, I added "Fix a BUG which can trigger when direct-IO is used with >>> AOE". Which kernel versions are affected? >> >> >> That BUG was added to the aoe driver in a commit that first appears in v3.7. It persists until this patch adds the code that handles the situation that the BUG was intended to detect. >> >> [ecashin@marino linux]$ git describe --contains --match 'v*' 69cf2d85de773d998798e47e3335b85e5645d157 >> v3.7-rc1~110^2~22 >> [ecashin@marino linux]$ >> > > ah, is it this? > > +bio_pageinc(struct bio *bio) > +{ > + struct bio_vec *bv; > + struct page *page; > + int i; > + > + bio_for_each_segment(bv, bio, i) { > + page = bv->bv_page; > + /* Non-zero page count for non-head members of > + * compound pages is no longer allowed by the kernel, > + * but this has never been seen here. > + */ > + if (unlikely(PageCompound(page))) > + if (compound_trans_head(page) != page) { > + pr_crit("page tail used for block I/O\n"); > + BUG(); > + } > > But get_page/put_page against a tail page of a compound page should > Just Work. The core MM will hunt down the head page and manipulate its > refcount. > > Perhaps the problem here is that AOE is diving under the covers and is > using low-level page refcount alterations: > > + atomic_inc(&page->_count); > > Why does AOE do this? It would be better if it were to use the > official published MM interfaces. If those interfaces need > alteration/augmentation then we can do that.
The get_page cannot be used when the _count is 0 (which is exactly when we need to increment it), because it will VM_BUG_ON.
/* * Getting a normal page or the head of a compound page * requires to already have an elevated page->_count. */ VM_BUG_ON(atomic_read(&page->_count) <= 0);
(And seeing the code reminds me that I forgot to include atomic_read in my last email's proposal. LKML + dinner isn't a good mix. ;)
-- Ed Cashin ecashin@coraid.com
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