Re: [PATCH] net: page_pool: fix refcounting issues with fragmented allocation

[Felix Fietkau <nbd@nbd.name>]

On 25.01.23 19:26, Alexander H Duyck wrote:
> On Wed, 2023-01-25 at 18:32 +0100, Felix Fietkau wrote:
>> On 25.01.23 18:11, Alexander H Duyck wrote:
>> > On Tue, 2023-01-24 at 22:30 +0100, Felix Fietkau wrote:
>> > > On 24.01.23 22:10, Alexander H Duyck wrote:
>> > > > On Tue, 2023-01-24 at 18:22 +0100, Felix Fietkau wrote:
>> > > > > On 24.01.23 15:11, Ilias Apalodimas wrote:
>> > > > > > Hi Felix,
>> > > > > > 
>> > > > > > ++cc Alexander and Yunsheng.
>> > > > > > 
>> > > > > > Thanks for the report
>> > > > > > 
>> > > > > > On Tue, 24 Jan 2023 at 14:43, Felix Fietkau <nbd@nbd.name> wrote:
>> > > > > > > 
>> > > > > > > While testing fragmented page_pool allocation in the mt76 driver, I was able
>> > > > > > > to reliably trigger page refcount underflow issues, which did not occur with
>> > > > > > > full-page page_pool allocation.
>> > > > > > > It appears to me, that handling refcounting in two separate counters
>> > > > > > > (page->pp_frag_count and page refcount) is racy when page refcount gets
>> > > > > > > incremented by code dealing with skb fragments directly, and
>> > > > > > > page_pool_return_skb_page is called multiple times for the same fragment.
>> > > > > > > 
>> > > > > > > Dropping page->pp_frag_count and relying entirely on the page refcount makes
>> > > > > > > these underflow issues and crashes go away.
>> > > > > > > 
>> > > > > > 
>> > > > > > This has been discussed here [1].  TL;DR changing this to page
>> > > > > > refcount might blow up in other colorful ways.  Can we look closer and
>> > > > > > figure out why the underflow happens?
>> > > > > I don't see how the approch taken in my patch would blow up. From what I 
>> > > > > can tell, it should be fairly close to how refcount is handled in 
>> > > > > page_frag_alloc. The main improvement it adds is to prevent it from 
>> > > > > blowing up if pool-allocated fragments get shared across multiple skbs 
>> > > > > with corresponding get_page and page_pool_return_skb_page calls.
>> > > > > 
>> > > > > - Felix
>> > > > > 
>> > > > 
>> > > > Do you have the patch available to review as an RFC? From what I am
>> > > > seeing it looks like you are underrunning on the pp_frag_count itself.
>> > > > I would suspect the issue to be something like starting with a bad
>> > > > count in terms of the total number of references, or deducing the wrong
>> > > > amount when you finally free the page assuming you are tracking your
>> > > > frag count using a non-atomic value in the driver.
>> > > The driver patches for page pool are here:
>> > > https://patchwork.kernel.org/project/linux-wireless/patch/64abb23f4867c075c19d704beaae5a0a2f8e8821.1673963374.git.lorenzo@kernel.org/
>> > > https://patchwork.kernel.org/project/linux-wireless/patch/68081e02cbe2afa2d35c8aa93194f0adddbd0f05.1673963374.git.lorenzo@kernel.org/
>> > > 
>> > > They are also applied in my mt76 tree at:
>> > > https://github.com/nbd168/wireless
>> > > 
>> > > - Felix
>> > 
>> > So one thing I am thinking is that we may be seeing an issue where we
>> > are somehow getting a mix of frag and non-frag based page pool pages.
>> > That is the only case I can think of where we might be underflowing
>> > negative. If you could add some additional debug info on the underflow
>> > WARN_ON case in page_pool_defrag_page that might be useful.
>> > Specifically I would be curious what the actual return value is. I'm
>> > assuming we are only hitting negative 1, but I would want to verify we
>> > aren't seeing something else.
>> I'll try to run some more tests soon. However, I think I found the piece 
>> of code that is incompatible with using pp_frag_count.
>> When receiving an A-MSDU packet (multiple MSDUs within a single 802.11 
>> packet), and it is not split by the hardware, a cfg80211 function 
>> extracts the individual MSDUs into separate skbs. In that case, a 
>> fragment can be shared across multiple skbs, and get_page is used to 
>> increase the refcount.
>> You can find this in net/wireless/util.c: ieee80211_amsdu_to_8023s (and 
>> its helper functions).
> 
> I'm not sure if it is problematic or not. Basically it is trading off
> by copying over the frags, calling get_page on each frag, and then
> using dev_kfree_skb to disassemble and release the pp_frag references.
> There should be other paths in the kernel that are doing something
> similar.
> 
>> This code also has a bug where it doesn't set pp_recycle on the newly 
>> allocated skb if the previous one has it, but that's a separate matter 
>> and fixing it doesn't make the crash go away.
> 
> Adding the recycle would cause this bug. So one thing we might be
> seeing is something like that triggering this error. Specifically if
> the page is taken via get_page when assembling the new skb then we
> cannot set the recycle flag in the new skb otherwise it will result in
> the reference undercount we are seeing. What we are doing is shifting
> the references away from the pp_frag_count to the page reference count
> in this case. If we set the pp_recycle flag then it would cause us to
> decrement pp_frag_count instead of the page reference count resulting
> in the underrun.
Couldn't leaving out the pp_recycle flag potentially lead to a case 
where the last user of the page drops it via page_frag_free instead of 
page_pool_return_skb_page? Is that valid?

>> Is there any way I can make that part of the code work with the current 
>> page pool frag implementation?
> 
> The current code should work. Basically as long as the references are
> taken w/ get_page and skb->pp_recycle is not set then we shouldn't run
> into this issue because the pp_frag_count will be dropped when the
> original skb is freed and the page reference count will be decremented
> when the new one is freed.
> 
> For page pool page fragments the main thing to keep in mind is that if
> pp_recycle is set it will update the pp_frag_count and if it is not
> then it will just decrement the page reference count.
What takes care of DMA unmap and other cleanup if the last reference to 
the page is dropped via page_frag_free?

- Felix