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[ Upstream commitef0ba05538] The kernel test robot reported a -5.8% performance regression on the "poll2" test of will-it-scale, and bisected it to commitd55564cfc2("x86: Make __put_user() generate an out-of-line call"). I didn't expect an out-of-line __put_user() to matter, because no normal core code should use that non-checking legacy version of user access any more. But I had overlooked the very odd poll() usage, which does a __put_user() to update the 'revents' values of the poll array. Now, Al Viro correctly points out that instead of updating just the 'revents' field, it would be much simpler to just copy the _whole_ pollfd entry, and then we could just use "copy_to_user()" on the whole array of entries, the same way we use "copy_from_user()" a few lines earlier to get the original values. But that is not what we've traditionally done, and I worry that threaded applications might be concurrently modifying the other fields of the pollfd array. So while Al's suggestion is simpler - and perhaps worth trying in the future - this instead keeps the "just update revents" model. To fix the performance regression, use the modern "unsafe_put_user()" instead of __put_user(), with the proper "user_write_access_begin()" guarding in place. This improves code generation enormously. Link: https://lore.kernel.org/lkml/20210107134723.GA28532@xsang-OptiPlex-9020/ Reported-by: kernel test robot <oliver.sang@intel.com> Tested-by: Oliver Sang <oliver.sang@intel.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: David Laight <David.Laight@aculab.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
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.