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

[Felix Fietkau <nbd@nbd.name>]

On 25.01.23 20:02, Alexander H Duyck wrote:
> On Wed, 2023-01-25 at 19:42 +0100, Felix Fietkau wrote:
>> 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?
> 
> No. What will happen is that when the pp_frag_count is exhausted the
> page will be unmapped and evicted from the page pool. When the page is
> then finally freed it will end up going back to the page allocator
> instead of page pool.
> 
> Basically the idea is that until pp_frag_count reaches 0 there will be
> at least 1 page reference held.
> 
>> > > 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
> 
> When the page is freed on the skb w/ pp_recycle set it will unmap the
> page and evict it from the page pool. Basically in these cases the page
> goes from the page pool back to the page allocator.
> 
> The general idea with this is that if we are using fragments that there
> will be enough of them floating around that if one or two frags have a
> temporeary detour through a non-recycling path that hopefully by the
> time the last fragment is freed the other instances holding the
> additional page reference will have let them go. If not then the page
> will go back to the page allocator and it will have to be replaced in
> the page pool.
Thanks for the explanation, it makes sense to me now. Unfortunately it 
also means that I have no idea what could cause this issue. I will 
finish my mt76 patch rework which gets rid of the pp vs non-pp 
allocation mix and re-run my tests to provide updated traces.

- Felix