lib/iov_iter: Document _copy_to_iter_mcsafe()
Add some theory of operation documentation to _copy_to_iter_mcsafe(). Reported-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tony Luck <tony.luck@intel.com> Link: http://lkml.kernel.org/r/153108276256.37979.1689794213845539316.stgit@dwillia2-desk3.amr.corp.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -596,6 +596,32 @@ static unsigned long memcpy_mcsafe_to_page(struct page *page, size_t offset,
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return ret;
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}
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/**
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* _copy_to_iter_mcsafe - copy to user with source-read error exception handling
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* @addr: source kernel address
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* @bytes: total transfer length
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* @iter: destination iterator
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*
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* The pmem driver arranges for filesystem-dax to use this facility via
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* dax_copy_to_iter() for protecting read/write to persistent memory.
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* Unless / until an architecture can guarantee identical performance
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* between _copy_to_iter_mcsafe() and _copy_to_iter() it would be a
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* performance regression to switch more users to the mcsafe version.
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*
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* Otherwise, the main differences between this and typical _copy_to_iter().
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*
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* * Typical tail/residue handling after a fault retries the copy
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* byte-by-byte until the fault happens again. Re-triggering machine
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* checks is potentially fatal so the implementation uses source
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* alignment and poison alignment assumptions to avoid re-triggering
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* hardware exceptions.
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*
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* * ITER_KVEC, ITER_PIPE, and ITER_BVEC can return short copies.
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* Compare to copy_to_iter() where only ITER_IOVEC attempts might return
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* a short copy.
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*
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* See MCSAFE_TEST for self-test.
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*/
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size_t _copy_to_iter_mcsafe(const void *addr, size_t bytes, struct iov_iter *i)
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{
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const char *from = addr;
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