| Message ID | 20241120103456.396577-4-linyunsheng@huawei.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | fix two bugs related to page_pool | expand |
On 20/11/2024 10:34 am, Yunsheng Lin wrote: > Skip dma sync operation for inflight pages before the > page_pool_destroy() returns to the driver as DMA API > expects to be called with a valid device bound to a > driver as mentioned in [1]. > > After page_pool_destroy() is called, the page is not > expected to be recycled back to pool->alloc cache and > dma sync operation is not needed when the page is not > recyclable or pool->ring is full, so only skip the dma > sync operation for the infilght pages by clearing the > pool->dma_sync under protection of rcu lock when page > is recycled to pool->ring to ensure that there is no > dma sync operation called after page_pool_destroy() is > returned. Something feels off here - either this is a micro-optimisation which I wouldn't really expect to be meaningful, or it means patch #2 doesn't actually do what it claims. If it really is possible to attempt to dma_sync a page *after* page_pool_inflight_unmap() has already reclaimed and unmapped it, that represents yet another DMA API lifecycle issue, which as well as being even more obviously incorrect usage-wise, could also still lead to the same crash (if the device is non-coherent). Otherwise, I don't imagine it's really worth worrying about optimising out syncs for any pages which happen to get naturally returned after page_pool_destroy() starts but before they're explicitly reclaimed. Realistically, the kinds of big server systems where reclaim takes an appreciable amount of time are going to be coherent and skipping syncs anyway. Thanks, Robin. > 1. https://lore.kernel.org/all/caf31b5e-0e8f-4844-b7ba-ef59ed13b74e@arm.com/ > CC: Robin Murphy <robin.murphy@arm.com> > CC: Alexander Duyck <alexander.duyck@gmail.com> > CC: Andrew Morton <akpm@linux-foundation.org> > CC: IOMMU <iommu@lists.linux.dev> > CC: MM <linux-mm@kvack.org> > Fixes: f71fec47c2df ("page_pool: make sure struct device is stable") > Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com> > --- > net/core/page_pool.c | 25 ++++++++++++++++++------- > 1 file changed, 18 insertions(+), 7 deletions(-) > > diff --git a/net/core/page_pool.c b/net/core/page_pool.c > index 33a314abbba4..0bde7c6c781a 100644 > --- a/net/core/page_pool.c > +++ b/net/core/page_pool.c > @@ -712,7 +712,8 @@ static void page_pool_return_page(struct page_pool *pool, netmem_ref netmem) > rcu_read_unlock(); > } > > -static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem) > +static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem, > + unsigned int dma_sync_size) > { > int ret; > /* BH protection not needed if current is softirq */ > @@ -723,10 +724,13 @@ static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem) > > if (!ret) { > recycle_stat_inc(pool, ring); > - return true; > + > + rcu_read_lock(); > + page_pool_dma_sync_for_device(pool, netmem, dma_sync_size); > + rcu_read_unlock(); > } > > - return false; > + return !ret; > } > > /* Only allow direct recycling in special circumstances, into the > @@ -779,10 +783,11 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem, > if (likely(__page_pool_page_can_be_recycled(netmem))) { > /* Read barrier done in page_ref_count / READ_ONCE */ > > - page_pool_dma_sync_for_device(pool, netmem, dma_sync_size); > - > - if (allow_direct && page_pool_recycle_in_cache(netmem, pool)) > + if (allow_direct && page_pool_recycle_in_cache(netmem, pool)) { > + page_pool_dma_sync_for_device(pool, netmem, > + dma_sync_size); > return 0; > + } > > /* Page found as candidate for recycling */ > return netmem; > @@ -845,7 +850,7 @@ void page_pool_put_unrefed_netmem(struct page_pool *pool, netmem_ref netmem, > > netmem = > __page_pool_put_page(pool, netmem, dma_sync_size, allow_direct); > - if (netmem && !page_pool_recycle_in_ring(pool, netmem)) { > + if (netmem && !page_pool_recycle_in_ring(pool, netmem, dma_sync_size)) { > /* Cache full, fallback to free pages */ > recycle_stat_inc(pool, ring_full); > page_pool_return_page(pool, netmem); > @@ -903,14 +908,18 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data, > > /* Bulk producer into ptr_ring page_pool cache */ > in_softirq = page_pool_producer_lock(pool); > + rcu_read_lock(); > for (i = 0; i < bulk_len; i++) { > if (__ptr_ring_produce(&pool->ring, data[i])) { > /* ring full */ > recycle_stat_inc(pool, ring_full); > break; > } > + page_pool_dma_sync_for_device(pool, (__force netmem_ref)data[i], > + -1); > } > recycle_stat_add(pool, ring, i); > + rcu_read_unlock(); > page_pool_producer_unlock(pool, in_softirq); > > /* Hopefully all pages was return into ptr_ring */ > @@ -1200,6 +1209,8 @@ void page_pool_destroy(struct page_pool *pool) > if (!page_pool_release(pool)) > return; > > + pool->dma_sync = false; > + > /* Paired with rcu lock in page_pool_napi_local() to enable clearing > * of pool->p.napi in page_pool_disable_direct_recycling() is seen > * before returning to driver to free the napi instance.
On 2024/11/21 0:17, Robin Murphy wrote: > On 20/11/2024 10:34 am, Yunsheng Lin wrote: >> Skip dma sync operation for inflight pages before the >> page_pool_destroy() returns to the driver as DMA API >> expects to be called with a valid device bound to a >> driver as mentioned in [1]. >> >> After page_pool_destroy() is called, the page is not >> expected to be recycled back to pool->alloc cache and >> dma sync operation is not needed when the page is not >> recyclable or pool->ring is full, so only skip the dma >> sync operation for the infilght pages by clearing the >> pool->dma_sync under protection of rcu lock when page >> is recycled to pool->ring to ensure that there is no >> dma sync operation called after page_pool_destroy() is >> returned. > > Something feels off here - either this is a micro-optimisation which I wouldn't really expect to be meaningful, or it means patch #2 doesn't actually do what it claims. If it really is possible to attempt to dma_sync a page *after* page_pool_inflight_unmap() has already reclaimed and unmapped it, that represents yet another DMA API lifecycle issue, which as well as being even more obviously incorrect usage-wise, could also still lead to the same crash (if the device is non-coherent). For a page_pool owned page, it mostly goes through the below steps: 1. page_pool calls buddy allocator API to allocate a page, call DMA mapping and sync_for_device API for it if its pool is empty. Or reuse the page in pool. 2. Driver calls the page_pool API to allocate the page, and pass the page to network stack after packet is dma'ed into the page and the sync_for_cpu API is called. 3. Network stack is done with page and called page_pool API to free the page. 4. page_pool releases the page back to buddy allocator if the page is not recyclable before doing the dma unmaping. Or do the sync_for_device and put the page in the its pool, the page might go through step 1 again if the driver calls the page_pool allocate API. The calling of dma mapping and dma sync API is controlled by pool->dma_map and pool->dma_sync respectively, the previous patch only clear pool->dma_map after doing the dma unmapping. This patch ensures that there is no dma_sync for recycle case of step 4 by clearing pool->dma_sync. The dma_sync skipping should also happen before page_pool_inflight_unmap() is called too because all the caller will see the clearing of pool->dma_sync after synchronize_rcu() and page_pool_inflight_unmap() is called after the same synchronize_rcu() in page_pool_destroy(). > > Otherwise, I don't imagine it's really worth worrying about optimising out syncs for any pages which happen to get naturally returned after page_pool_destroy() starts but before they're explicitly reclaimed. Realistically, the kinds of big server systems where reclaim takes an appreciable amount of time are going to be coherent and skipping syncs anyway. The skipping is about skipping the dma sync for those inflight pages, I should make it clearer that the skipping happens before the calling of page_pool_inflight_unmap() instead of page_pool_destroy() in the commit log. >
On 21/11/2024 8:04 am, Yunsheng Lin wrote: > On 2024/11/21 0:17, Robin Murphy wrote: >> On 20/11/2024 10:34 am, Yunsheng Lin wrote: >>> Skip dma sync operation for inflight pages before the >>> page_pool_destroy() returns to the driver as DMA API >>> expects to be called with a valid device bound to a >>> driver as mentioned in [1]. >>> >>> After page_pool_destroy() is called, the page is not >>> expected to be recycled back to pool->alloc cache and >>> dma sync operation is not needed when the page is not >>> recyclable or pool->ring is full, so only skip the dma >>> sync operation for the infilght pages by clearing the >>> pool->dma_sync under protection of rcu lock when page >>> is recycled to pool->ring to ensure that there is no >>> dma sync operation called after page_pool_destroy() is >>> returned. >> >> Something feels off here - either this is a micro-optimisation which I wouldn't really expect to be meaningful, or it means patch #2 doesn't actually do what it claims. If it really is possible to attempt to dma_sync a page *after* page_pool_inflight_unmap() has already reclaimed and unmapped it, that represents yet another DMA API lifecycle issue, which as well as being even more obviously incorrect usage-wise, could also still lead to the same crash (if the device is non-coherent). > > For a page_pool owned page, it mostly goes through the below steps: > 1. page_pool calls buddy allocator API to allocate a page, call DMA mapping > and sync_for_device API for it if its pool is empty. Or reuse the page in > pool. > > 2. Driver calls the page_pool API to allocate the page, and pass the page > to network stack after packet is dma'ed into the page and the sync_for_cpu > API is called. > > 3. Network stack is done with page and called page_pool API to free the page. > > 4. page_pool releases the page back to buddy allocator if the page is not > recyclable before doing the dma unmaping. Or do the sync_for_device > and put the page in the its pool, the page might go through step 1 > again if the driver calls the page_pool allocate API. > > The calling of dma mapping and dma sync API is controlled by pool->dma_map > and pool->dma_sync respectively, the previous patch only clear pool->dma_map > after doing the dma unmapping. This patch ensures that there is no dma_sync > for recycle case of step 4 by clearing pool->dma_sync. But *why* does it want to ensure that? Is there some possible race where one thread can attempt to sync and recycle a page while another thread is attempting to unmap and free it, such that you can't guarantee the correctness of dma_sync calls after page_pool_inflight_unmap() has started, and skipping them is a workaround for that? If so, then frankly I think that would want solving properly, but at the very least this change would need to come before patch #2. If not, and this is just some attempt at performance micro-optimisation, then I'd be keen to see the numbers to justify it, since I struggle to imagine it being worth the bother while already in the process of spending whole seconds scanning memory... Thanks, Robin. > The dma_sync skipping should also happen before page_pool_inflight_unmap() > is called too because all the caller will see the clearing of pool->dma_sync > after synchronize_rcu() and page_pool_inflight_unmap() is called after > the same synchronize_rcu() in page_pool_destroy(). > >> >> Otherwise, I don't imagine it's really worth worrying about optimising out syncs for any pages which happen to get naturally returned after page_pool_destroy() starts but before they're explicitly reclaimed. Realistically, the kinds of big server systems where reclaim takes an appreciable amount of time are going to be coherent and skipping syncs anyway. > > The skipping is about skipping the dma sync for those inflight pages, > I should make it clearer that the skipping happens before the calling > of page_pool_inflight_unmap() instead of page_pool_destroy() in the > commit log. > >> >
On 2024/11/21 21:44, Robin Murphy wrote: > On 21/11/2024 8:04 am, Yunsheng Lin wrote: >> On 2024/11/21 0:17, Robin Murphy wrote: >>> On 20/11/2024 10:34 am, Yunsheng Lin wrote: >>>> Skip dma sync operation for inflight pages before the >>>> page_pool_destroy() returns to the driver as DMA API >>>> expects to be called with a valid device bound to a >>>> driver as mentioned in [1]. >>>> >>>> After page_pool_destroy() is called, the page is not >>>> expected to be recycled back to pool->alloc cache and >>>> dma sync operation is not needed when the page is not >>>> recyclable or pool->ring is full, so only skip the dma >>>> sync operation for the infilght pages by clearing the >>>> pool->dma_sync under protection of rcu lock when page >>>> is recycled to pool->ring to ensure that there is no >>>> dma sync operation called after page_pool_destroy() is >>>> returned. >>> >>> Something feels off here - either this is a micro-optimisation which I wouldn't really expect to be meaningful, or it means patch #2 doesn't actually do what it claims. If it really is possible to attempt to dma_sync a page *after* page_pool_inflight_unmap() has already reclaimed and unmapped it, that represents yet another DMA API lifecycle issue, which as well as being even more obviously incorrect usage-wise, could also still lead to the same crash (if the device is non-coherent). >> >> For a page_pool owned page, it mostly goes through the below steps: >> 1. page_pool calls buddy allocator API to allocate a page, call DMA mapping >> and sync_for_device API for it if its pool is empty. Or reuse the page in >> pool. >> >> 2. Driver calls the page_pool API to allocate the page, and pass the page >> to network stack after packet is dma'ed into the page and the sync_for_cpu >> API is called. >> >> 3. Network stack is done with page and called page_pool API to free the page. >> >> 4. page_pool releases the page back to buddy allocator if the page is not >> recyclable before doing the dma unmaping. Or do the sync_for_device >> and put the page in the its pool, the page might go through step 1 >> again if the driver calls the page_pool allocate API. >> >> The calling of dma mapping and dma sync API is controlled by pool->dma_map >> and pool->dma_sync respectively, the previous patch only clear pool->dma_map >> after doing the dma unmapping. This patch ensures that there is no dma_sync >> for recycle case of step 4 by clearing pool->dma_sync. > > But *why* does it want to ensure that? Is there some possible race where one thread can attempt to sync and recycle a page while another thread is attempting to unmap and free it, such that you can't guarantee the correctness of dma_sync calls after page_pool_inflight_unmap() has started, and skipping them is a workaround for that? If so, then frankly I think that would want solving properly, but at the very least this change would need to come before patch #2. The racing window is something like below. page_pool_destroy() and page_pool_put_page() can be called concurrently, patch 2 only use a spinlock to synchronise page_pool_inflight_unmap() with page_pool_return_page() called by page_pool_put_page() to avoid concurrent dma unmapping, there is no synchronization between page_pool_destroy() and page_pool_dma_sync_for_device() called by page_pool_put_page(): CPU0 CPU1 . . page_pool_destroy() page_pool_put_page() . . synchronize_rcu() . . . page_pool_inflight_unmap() . . . . __page_pool_put_page() . . . page_pool_dma_sync_for_device() . . After this patch, page_pool_dma_sync_for_device() is protected by rcu lock and pool->dma_sync is cleared before synchronize_rcu and page_pool_inflight_unmap() is called after synchronize_rcu to ensure page_pool_dma_sync_for_device() will not call dma sync API after synchronize_rcu(): CPU0 CPU1 . . page_pool_destroy() CPU page_pool_put_page() CPU . . pool->dma_sync = false . . . synchronize_rcu() . . . page_pool_inflight_unmap() . . . . page_pool_recycle_in_ring() . . . rcu_read_lock() . page_pool_dma_sync_for_device() . rcu_read_unlock() Previously patch 2&3 was combined as one patch, this version splits it out to make it more reviewable. I am not sure if it matters that much about the patch order as the fix doesn't seem to be completed unless both patches are included.
diff --git a/net/core/page_pool.c b/net/core/page_pool.c index 33a314abbba4..0bde7c6c781a 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -712,7 +712,8 @@ static void page_pool_return_page(struct page_pool *pool, netmem_ref netmem) rcu_read_unlock(); } -static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem) +static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem, + unsigned int dma_sync_size) { int ret; /* BH protection not needed if current is softirq */ @@ -723,10 +724,13 @@ static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem) if (!ret) { recycle_stat_inc(pool, ring); - return true; + + rcu_read_lock(); + page_pool_dma_sync_for_device(pool, netmem, dma_sync_size); + rcu_read_unlock(); } - return false; + return !ret; } /* Only allow direct recycling in special circumstances, into the @@ -779,10 +783,11 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem, if (likely(__page_pool_page_can_be_recycled(netmem))) { /* Read barrier done in page_ref_count / READ_ONCE */ - page_pool_dma_sync_for_device(pool, netmem, dma_sync_size); - - if (allow_direct && page_pool_recycle_in_cache(netmem, pool)) + if (allow_direct && page_pool_recycle_in_cache(netmem, pool)) { + page_pool_dma_sync_for_device(pool, netmem, + dma_sync_size); return 0; + } /* Page found as candidate for recycling */ return netmem; @@ -845,7 +850,7 @@ void page_pool_put_unrefed_netmem(struct page_pool *pool, netmem_ref netmem, netmem = __page_pool_put_page(pool, netmem, dma_sync_size, allow_direct); - if (netmem && !page_pool_recycle_in_ring(pool, netmem)) { + if (netmem && !page_pool_recycle_in_ring(pool, netmem, dma_sync_size)) { /* Cache full, fallback to free pages */ recycle_stat_inc(pool, ring_full); page_pool_return_page(pool, netmem); @@ -903,14 +908,18 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data, /* Bulk producer into ptr_ring page_pool cache */ in_softirq = page_pool_producer_lock(pool); + rcu_read_lock(); for (i = 0; i < bulk_len; i++) { if (__ptr_ring_produce(&pool->ring, data[i])) { /* ring full */ recycle_stat_inc(pool, ring_full); break; } + page_pool_dma_sync_for_device(pool, (__force netmem_ref)data[i], + -1); } recycle_stat_add(pool, ring, i); + rcu_read_unlock(); page_pool_producer_unlock(pool, in_softirq); /* Hopefully all pages was return into ptr_ring */ @@ -1200,6 +1209,8 @@ void page_pool_destroy(struct page_pool *pool) if (!page_pool_release(pool)) return; + pool->dma_sync = false; + /* Paired with rcu lock in page_pool_napi_local() to enable clearing * of pool->p.napi in page_pool_disable_direct_recycling() is seen * before returning to driver to free the napi instance.
Skip dma sync operation for inflight pages before the page_pool_destroy() returns to the driver as DMA API expects to be called with a valid device bound to a driver as mentioned in [1]. After page_pool_destroy() is called, the page is not expected to be recycled back to pool->alloc cache and dma sync operation is not needed when the page is not recyclable or pool->ring is full, so only skip the dma sync operation for the infilght pages by clearing the pool->dma_sync under protection of rcu lock when page is recycled to pool->ring to ensure that there is no dma sync operation called after page_pool_destroy() is returned. 1. https://lore.kernel.org/all/caf31b5e-0e8f-4844-b7ba-ef59ed13b74e@arm.com/ CC: Robin Murphy <robin.murphy@arm.com> CC: Alexander Duyck <alexander.duyck@gmail.com> CC: Andrew Morton <akpm@linux-foundation.org> CC: IOMMU <iommu@lists.linux.dev> CC: MM <linux-mm@kvack.org> Fixes: f71fec47c2df ("page_pool: make sure struct device is stable") Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com> --- net/core/page_pool.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-)