On Wed, 2 Oct 2024 at 09:46, Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx> wrote: > > Hi Paolo, > > Thanks for taking the time. > > On Tue, 1 Oct 2024 at 16:32, Paolo Abeni <pabeni@xxxxxxxxxx> wrote: > > > > On 9/25/24 09:57, Yunsheng Lin wrote: > > > Networking driver with page_pool support may hand over page > > > still with dma mapping to network stack and try to reuse that > > > page after network stack is done with it and passes it back > > > to page_pool to avoid the penalty of dma mapping/unmapping. > > > With all the caching in the network stack, some pages may be > > > held in the network stack without returning to the page_pool > > > soon enough, and with VF disable causing the driver unbound, > > > the page_pool does not stop the driver from doing it's > > > unbounding work, instead page_pool uses workqueue to check > > > if there is some pages coming back from the network stack > > > periodically, if there is any, it will do the dma unmmapping > > > related cleanup work. > > > > > > As mentioned in [1], attempting DMA unmaps after the driver > > > has already unbound may leak resources or at worst corrupt > > > memory. Fundamentally, the page pool code cannot allow DMA > > > mappings to outlive the driver they belong to. > > > > > > Currently it seems there are at least two cases that the page > > > is not released fast enough causing dma unmmapping done after > > > driver has already unbound: > > > 1. ipv4 packet defragmentation timeout: this seems to cause > > > delay up to 30 secs. > > > 2. skb_defer_free_flush(): this may cause infinite delay if > > > there is no triggering for net_rx_action(). > > > > > > In order not to do the dma unmmapping after driver has already > > > unbound and stall the unloading of the networking driver, add > > > the pool->items array to record all the pages including the ones > > > which are handed over to network stack, so the page_pool can > > > do the dma unmmapping for those pages when page_pool_destroy() > > > is called. As the pool->items need to be large enough to avoid > > > performance degradation, add a 'item_full' stat to indicate the > > > allocation failure due to unavailability of pool->items. > > > > This looks really invasive, with room for potentially large performance > > regressions or worse. At very least it does not look suitable for net. > > Perhaps, and you are right we need to measure performance before > pulling it but... > > > > > Is the problem only tied to VFs drivers? It's a pity all the page_pool > > users will have to pay a bill for it... > > It's not. The problem happens when an SKB has been scheduled for > recycling and has already been mapped via page_pool. If the driver > disappears in the meantime, Apologies, this wasn't correct. It's the device that has to disappear not the driver > page_pool will free all the packets it > holds in its private rings (both slow and fast), but is not in control > of the SKB anymore. So any packets coming back for recycling *after* > that point cannot unmap memory properly. > > As discussed this can either lead to memory corruption and resource > leaking, or worse as seen in the bug report panics. I am fine with > this going into -next, but it really is a bugfix, although I am not > 100% sure that the Fixes: tag in the current patch is correct. > > Thanks > /Ilias > > > > /P > >
