linux-kernel - Re: BUG: KCSAN: data-race in folio_batch_move_lru / mpage_read_end

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Date:   Tue, 19 Sep 2023 13:44:22 +0200
From:   Mirsad Todorovac <mirsad.todorovac@....hr>
To:     Matthew Wilcox <willy@...radead.org>,
        Mirsad Todorovac <mirsad.todorovac@....unizg.hr>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        linux-nvme@...ts.infradead.org
Subject: Re: BUG: KCSAN: data-race in folio_batch_move_lru / mpage_read_end_io

On 9/18/2023 4:53 PM, Matthew Wilcox wrote:

> On Mon, Sep 18, 2023 at 02:15:05PM +0200, Mirsad Todorovac wrote:
>>> This is what I'm currently running with, and it doesn't trigger.
>>> I'd expect it to if we were going to hit the KCSAN bug.
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 0c5be12f9336..d22e8798c326 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -4439,6 +4439,7 @@ struct page *__alloc_pages(gfp_t gfp, unsigned int order, int preferred_nid,
>>>    	page = __alloc_pages_slowpath(alloc_gfp, order, &ac);
>>>    out:
>>> +	VM_BUG_ON_PAGE(page && (page->flags & (PAGE_FLAGS_CHECK_AT_PREP &~ (1 << PG_head))), page);
>>>    	if (memcg_kmem_online() && (gfp & __GFP_ACCOUNT) && page &&
>>>    	    unlikely(__memcg_kmem_charge_page(page, gfp, order) != 0)) {
>>>    		__free_pages(page, order);
>> Hi,
>>
>> Caught another instance of this bug involving folio_batch_move_lru: I don't seem that I can make it
>> happen reliably by the nature of the data racing conditions if I understood them well.
> Were you running with this patch at the time, or was this actually
> vanilla?  The problem is that, if my diagnosis is correct, both of the
> tasks mentioned are victims; we have a prematurely freed page.  While
> btrfs is clearly a user, it may not be btrfs's fault that the
> page was also allocated as an anon page.
>
> I'm trying to gather more data, and running with this patch will give
> us more -- because it'll dump the entire struct page instead of just
> the page->flags, like KCSAN is currently doing.

Hi, Mr. Matthew,

Yes, I am using "vanilla with your VM_BUG_ON_PAGE()" patch all the time, 
as it seems non-disruptive and I am hoping to catch this spurious page 
alloc.

Best regards, Mirsad Todorovac