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

[Felix Fietkau <nbd@nbd.name>]

On 25.01.23 20:10, Felix Fietkau wrote:
> 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.
Here's the updated mt76 page pool support commit:
https://github.com/nbd168/wireless/commit/923cdab6d4c92a0acb3536b3b0cc4af9fee7c808

And here is the trace that I'm getting with 6.1:
https://nbd.name/p/a16957f2

If you have any debug patch you'd like me to test, please let me know.

- Felix