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__napi_alloc_skb() is napi_alloc_skb() with the added flexibility of choosing gfp_mask. This is a NAPI function, so GFP_ATOMIC is implied. The only practical choice the caller has is whether to set __GFP_NOWARN. But that's a false choice, too, allocation failures in atomic context will happen, and printing warnings in logs, effectively for a packet drop, is both too much and very likely non-actionable. This leads me to a conclusion that most uses of napi_alloc_skb() are simply misguided, and should use __GFP_NOWARN in the first place. We also have a "standard" way of reporting allocation failures via the queue stat API (qstats::rx-alloc-fail). The direct motivation for this patch is that one of the drivers used at Meta calls napi_alloc_skb() (so prior to this patch without __GFP_NOWARN), and the resulting OOM warning is the top networking warning in our fleet. Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com> Reviewed-by: Simon Horman <horms@kernel.org> Reviewed-by: Eric Dumazet <edumazet@google.com> Link: https://lore.kernel.org/r/20240327040213.3153864-1-kuba@kernel.org Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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2.2 KiB
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44 lines
2.2 KiB
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Page fragments
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==============
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A page fragment is an arbitrary-length arbitrary-offset area of memory
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which resides within a 0 or higher order compound page. Multiple
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fragments within that page are individually refcounted, in the page's
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reference counter.
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The page_frag functions, page_frag_alloc and page_frag_free, provide a
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simple allocation framework for page fragments. This is used by the
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network stack and network device drivers to provide a backing region of
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memory for use as either an sk_buff->head, or to be used in the "frags"
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portion of skb_shared_info.
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In order to make use of the page fragment APIs a backing page fragment
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cache is needed. This provides a central point for the fragment allocation
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and tracks allows multiple calls to make use of a cached page. The
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advantage to doing this is that multiple calls to get_page can be avoided
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which can be expensive at allocation time. However due to the nature of
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this caching it is required that any calls to the cache be protected by
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either a per-cpu limitation, or a per-cpu limitation and forcing interrupts
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to be disabled when executing the fragment allocation.
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The network stack uses two separate caches per CPU to handle fragment
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allocation. The netdev_alloc_cache is used by callers making use of the
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netdev_alloc_frag and __netdev_alloc_skb calls. The napi_alloc_cache is
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used by callers of the __napi_alloc_frag and napi_alloc_skb calls. The
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main difference between these two calls is the context in which they may be
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called. The "netdev" prefixed functions are usable in any context as these
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functions will disable interrupts, while the "napi" prefixed functions are
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only usable within the softirq context.
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Many network device drivers use a similar methodology for allocating page
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fragments, but the page fragments are cached at the ring or descriptor
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level. In order to enable these cases it is necessary to provide a generic
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way of tearing down a page cache. For this reason __page_frag_cache_drain
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was implemented. It allows for freeing multiple references from a single
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page via a single call. The advantage to doing this is that it allows for
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cleaning up the multiple references that were added to a page in order to
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avoid calling get_page per allocation.
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Alexander Duyck, Nov 29, 2016.
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