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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Rafael Aquini <aquini@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org
Subject: Re: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference
References: <20200922184838.978540-1-aquini@redhat.com>
        <878sd1qllb.fsf@yhuang-dev.intel.com>
        <20200923043459.GL795820@optiplex-lnx>
        <87sgb9oz1u.fsf@yhuang-dev.intel.com>
        <20200923130138.GM795820@optiplex-lnx>
        <87blhwng5f.fsf@yhuang-dev.intel.com>
        <20200924020928.GC1023012@optiplex-lnx>
        <877dsjessq.fsf@yhuang-dev.intel.com>
        <20200924063038.GD1023012@optiplex-lnx>
Date:   Thu, 24 Sep 2020 14:57:20 +0800
In-Reply-To: <20200924063038.GD1023012@optiplex-lnx> (Rafael Aquini's message
        of "Thu, 24 Sep 2020 02:30:38 -0400")
Message-ID: <87y2kzd5m7.fsf@yhuang-dev.intel.com>
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Rafael Aquini <aquini@redhat.com> writes:

>> 
>> If there's a race, we should fix the race.  But the code path for
>> swapcache insertion is,
>> 
>> add_to_swap()
>>   get_swap_page() /* Return if fails to allocate */
>>   add_to_swap_cache()
>>     SetPageSwapCache()
>> 
>> While the code path to split THP is,
>> 
>> split_huge_page_to_list()
>>   if PageSwapCache()
>>     split_swap_cluster()
>> 
>> Both code paths are protected by the page lock.  So there should be some
>> other reasons to trigger the bug.
>
> As mentioned above, no they seem to not be protected (at least, not the
> same page, depending on the case). While add_to_swap() will assure a 
> page_lock on the compound head, split_huge_page_to_list() does not.
>

int split_huge_page_to_list(struct page *page, struct list_head *list)
{
	struct page *head = compound_head(page);
	struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
	struct deferred_split *ds_queue = get_deferred_split_queue(head);
	struct anon_vma *anon_vma = NULL;
	struct address_space *mapping = NULL;
	int count, mapcount, extra_pins, ret;
	unsigned long flags;
	pgoff_t end;

	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
	VM_BUG_ON_PAGE(!PageLocked(head), head);

I found there's page lock checking in split_huge_page_to_list().

Best Regards,
Huang, Ying