linux-kernel - Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

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Message-ID: <20210308204731.GE3479805@casper.infradead.org>
Date:   Mon, 8 Mar 2021 20:47:31 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        Zhou Guanghui <zhouguanghui1@...wei.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        hannes@...xchg.org, hughd@...gle.com,
        kirill.shutemov@...ux.intel.com, npiggin@...il.com, ziy@...dia.com,
        wangkefeng.wang@...wei.com, guohanjun@...wei.com,
        dingtianhong@...wei.com, chenweilong@...wei.com,
        rui.xiang@...wei.com
Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

On Mon, Mar 08, 2021 at 12:42:27PM -0800, Andrew Morton wrote:
> On Mon, 8 Mar 2021 09:41:38 +0100 Michal Hocko <mhocko@...e.com> wrote:
> 
> > On Fri 05-03-21 15:58:40, Andrew Morton wrote:
> > > On Fri, 5 Mar 2021 12:52:52 +0100 Michal Hocko <mhocko@...e.com> wrote:
> > > 
> > > > On Thu 04-03-21 07:40:53, Zhou Guanghui wrote:
> > > > > As described in the split_page function comment, for the non-compound
> > > > > high order page, the sub-pages must be freed individually. If the
> > > > > memcg of the fisrt page is valid, the tail pages cannot be uncharged
> > > > > when be freed.
> > > > > 
> > > > > For example, when alloc_pages_exact is used to allocate 1MB continuous
> > > > > physical memory, 2MB is charged(kmemcg is enabled and __GFP_ACCOUNT is
> > > > > set). When make_alloc_exact free the unused 1MB and free_pages_exact
> > > > > free the applied 1MB, actually, only 4KB(one page) is uncharged.
> > > > > 
> > > > > Therefore, the memcg of the tail page needs to be set when split page.
> > > > > 
> > > > 
> > > > As already mentioned there are at least two explicit users of
> > > > __GFP_ACCOUNT with alloc_exact_pages added recently. It would be good to
> > > > mention that explicitly and maybe even mention 7efe8ef274024 resp.
> > > > c419621873713 so that it is clear this is not just a theoretical issue.
> > > 
> > > I added
> > > 
> > > : Michel:
> > > : 
> > > : There are at least two explicit users of __GFP_ACCOUNT with
> > > : alloc_exact_pages added recently.  See 7efe8ef274024 ("KVM: arm64:
> > > : Allocate stage-2 pgd pages with GFP_KERNEL_ACCOUNT") and c419621873713
> > > : ("KVM: s390: Add memcg accounting to KVM allocations"), so this is not
> > > : just a theoretical issue.
> > > 
> > > And should we cc:stable on this one?
> > 
> > Somebody more familiar with iommu dma allocation layer should have a
> > look as well (__iommu_dma_alloc_pages) so that we know whether there are
> > kernels outside of the above two ones mentioned above that need a fix.
> > But in general this sounds like a good fit for the stable tree.
> 
> OK.  I reversed the order of these two patches so we don't need to
> burden -stable with a cosmetic rename.

Eek, no.

The alloc_pages_exact() is done to pages that _aren't_ compound.
So you have to pass the number of pages to the memcg split function,
because a non-compound page doesn't know the size of its allocation.