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Talal Ahmad says: ==================== Accurate Memory Charging For MSG_ZEROCOPY This series improves the accuracy of msg_zerocopy memory accounting. At present, when msg_zerocopy is used memory is charged twice for the data - once when user space allocates it, and then again within __zerocopy_sg_from_iter. The memory charging in the kernel is excessive because data is held in user pages and is never actually copied to skb fragments. This leads to incorrectly inflated memory statistics for programs passing MSG_ZEROCOPY. We reduce this inaccuracy by introducing the notion of "pure" zerocopy SKBs - where all the frags in the SKB are backed by pinned userspace pages, and none are backed by copied pages. For such SKBs, tracked via the new SKBFL_PURE_ZEROCOPY flag, we elide sk_mem_charge/uncharge calls, leading to more accurate accounting. However, SKBs can also be coalesced by the stack at present, potentially leading to "impure" SKBs. We restrict this coalescing so it can only happen within the sendmsg() system call itself, for the most recently allocated SKB. While this can lead to a small degree of double-charging of memory, this case does not arise often in practice for workloads that set MSG_ZEROCOPY. Testing verified that memory usage in the kernel is lowered. Instrumentation with counters also showed that accounting at time charging and uncharging is balanced. ==================== Link: https://lore.kernel.org/r/20211030020542.3870542-1-mailtalalahmad@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.