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Since tmpfs THP was supported in 4.8, hugetlbfs is not the only filesystem with huge page support anymore. tmpfs can use huge page via THP when mounting by "huge=" mount option. When applications use huge page on hugetlbfs, it just need check the filesystem magic number, but it is not enough for tmpfs. Make stat.st_blksize return huge page size if it is mounted by appropriate "huge=" option to give applications a hint to optimize the behavior with THP. Some applications may not do wisely with THP. For example, QEMU may mmap file on non huge page aligned hint address with MAP_FIXED, which results in no pages are PMD mapped even though THP is used. Some applications may mmap file with non huge page aligned offset. Both behaviors make THP pointless. statfs.f_bsize still returns 4KB for tmpfs since THP could be split, and it also may fallback to 4KB page silently if there is not enough huge page. Furthermore, different f_bsize makes max_blocks and free_blocks calculation harder but without too much benefit. Returning huge page size via stat.st_blksize sounds good enough. Since PUD size huge page for THP has not been supported, now it just returns HPAGE_PMD_SIZE. Hugh said: : Sorry, I have no enthusiasm for this patch; but do I feel strongly : enough to override you and everyone else to NAK it? No, I don't feel : that strongly, maybe st_blksize isn't worth arguing over. : : We did look at struct stat when designing huge tmpfs, to see if there : were any fields that should be adjusted for it; but concluded none. : Yes, it would sometimes be nice to have a quickly accessible indicator : for when tmpfs has been mounted huge (scanning /proc/mounts for options : can be tiresome, agreed); but since tmpfs tries to supply huge (or not) : pages transparently, no difference seemed right. : : So, because st_blksize is a not very useful field of struct stat, with : "size" in the name, we're going to put HPAGE_PMD_SIZE in there instead : of PAGE_SIZE, if the tmpfs was mounted with one of the huge "huge" : options (force or always, okay; within_size or advise, not so much). : Though HPAGE_PMD_SIZE is no more its "preferred I/O size" or "blocksize : for file system I/O" than PAGE_SIZE was. : : Which we can expect to speed up some applications and disadvantage : others, depending on how they interpret st_blksize: just like if we : changed it in the same way on non-huge tmpfs. (Did I actually try : changing st_blksize early on, and find it broke something? If so, I've : now forgotten what, and a search through commit messages didn't find : it; but I guess we'll find out soon enough.) : : If there were an mstat() syscall, returning a field "preferred : alignment", then we could certainly agree to put HPAGE_PMD_SIZE in : there; but in stat()'s st_blksize? And what happens when (in future) : mm maps this or that hard-disk filesystem's blocks with a pmd mapping - : should that filesystem then advertise a bigger st_blksize, despite the : same disk layout as before? What happens with DAX? : : And this change is not going to help the QEMU suboptimality that : brought you here (or does QEMU align mmaps according to st_blksize?). : QEMU ought to work well with kernels without this change, and kernels : with this change; and I hope it can easily deal with both by avoiding : that use of MAP_FIXED which prevented the kernel's intended alignment. [akpm@linux-foundation.org: remove unneeded `else'] Link: http://lkml.kernel.org/r/1524665633-83806-1-git-send-email-yang.shi@linux.alibaba.com Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> Suggested-by: Christoph Hellwig <hch@infradead.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Hugh Dickins <hughd@google.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.