linux-kernel - Re: [Resend PATCHv4 1/1] mm: fix incorrect vbq reference in purge_fragmented

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Date: Thu, 13 Jun 2024 17:23:26 +0800
From: Zhaoyang Huang <huangzhaoyang@...il.com>
To: hailong liu <hailong.liu@...o.com>
Cc: Baoquan He <bhe@...hat.com>, Uladzislau Rezki <urezki@...il.com>, 
	"zhaoyang.huang" <zhaoyang.huang@...soc.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Hellwig <hch@...radead.org>, Lorenzo Stoakes <lstoakes@...il.com>, 
	Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org, steve.kang@...soc.com
Subject: Re: [Resend PATCHv4 1/1] mm: fix incorrect vbq reference in purge_fragmented_block

On Thu, Jun 13, 2024 at 5:11 PM hailong liu <hailong.liu@...o.com> wrote:
>
> On Thu, 13. Jun 16:41, Baoquan He wrote:
> > On 06/12/24 at 01:27pm, Uladzislau Rezki wrote:
> > > On Wed, Jun 12, 2024 at 10:00:14AM +0800, Zhaoyang Huang wrote:
> > > > On Wed, Jun 12, 2024 at 2:16 AM Uladzislau Rezki <urezki@...il.com> wrote:
> > > > >
> > > > > >
> > > > > > Sorry to bother you again. Are there any other comments or new patch
> > > > > > on this which block some test cases of ANDROID that only accept ACKed
> > > > > > one on its tree.
> > > > > >
> > > > > I have just returned from vacation. Give me some time to review your
> > > > > patch. Meanwhile, do you have a reproducer? So i would like to see how
> > > > > i can trigger an issue that is in question.
> > > > This bug arises from an system wide android test which has been
> > > > reported by many vendors. Keep mount/unmount an erofs partition is
> > > > supposed to be a simple reproducer. IMO, the logic defect is obvious
> > > > enough to be found by code review.
> > > >
> > > Baoquan, any objection about this v4?
> > >
> > > Your proposal about inserting a new vmap-block based on it belongs
> > > to, i.e. not per-this-cpu, should fix an issue. The problem is that
> > > such way does __not__ pre-load a current CPU what is not good.
> >
> > With my understand, when we start handling to insert vb to vbq->xa and
> > vbq->free, the vmap_area allocation has been done, it doesn't impact the
> > CPU preloading when adding it into which CPU's vbq->free, does it?
> >
> > Not sure if I miss anything about the CPU preloading.
> >
> >
>
> IIUC, if vb put by hashing funcation. and the following scenario may occur:
>
> A kthread limit on CPU_x and continuously calls vm_map_ram()
> The 1 call vm_map_ram(): no vb in cpu_x->free, so
> CPU_0->vb
> CPU_1
> ...
> CPU_x
>
> The 2 call vm_map_ram(): no vb in cpu_x->free, so
> CPU_0->vb
> CPU_1->vb
> ...
> CPU_x
Yes, this could make the per_cpu vbq meaningless and the VMALLOC area
be abnormally consumed(like 8KB in 4MB for each allocation)
>
> --
> help you, help me,
> Hailong.