Messages in this thread | | | Subject | Re: [RFC PATCH] fs: introduce ST_HUGE flag and set it to tmpfs and hugetlbfs | From | Yang Shi <> | Date | Wed, 18 Apr 2018 11:18:25 -0700 |
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On 4/18/18 3:27 AM, Christoph Hellwig wrote: > On Wed, Apr 18, 2018 at 05:08:13AM +0800, Yang Shi wrote: >> 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. So, introduce >> ST_HUGE flag to statfs if super block has SB_HUGE set which indicates >> huge page is supported on the specific filesystem. >> >> Some applications could benefit from this change, for example QEMU. >> When use mmap file as guest VM backend memory, QEMU typically mmap the >> file size plus one extra page. If the file is on hugetlbfs the extra >> page is huge page size (i.e. 2MB), but it is still 4KB on tmpfs even >> though THP is enabled. tmpfs THP requires VMA is huge page aligned, so >> if 4KB page is used THP will not be used at all. The below /proc/meminfo >> fragment shows the THP use of QEMU with 4K page: >> >> ShmemHugePages: 679936 kB >> ShmemPmdMapped: 0 kB >> >> With ST_HUGE flag, QEMU can get huge page, then /proc/meminfo looks >> like: >> >> ShmemHugePages: 77824 kB >> ShmemPmdMapped: 6144 kB >> >> With this flag, the applications can know if huge page is supported on >> the filesystem then optimize the behavior of the applications >> accordingly. Although the similar function can be implemented in >> applications by traversing the mount options, it looks more convenient >> if kernel can provide such flag. >> >> Even though ST_HUGE is set, f_bsize still returns 4KB for tmpfs since >> THP could be split, and it also my fallback to 4KB page silently if >> there is not enough huge page. > Seems like your should report it through the st_blksize field of struct > stat then, instead of introducing a not very useful binary field then.
Yes, thanks for the suggestion. I did think about it before I went with the new flag. Not like hugetlb, THP will *not* guarantee huge page is used all the time, it may fallback to regular 4K page or may get split. I'm not sure how the applications use f_bsize field, it might break existing applications and the value might be abused by applications to have counter optimization. So, IMHO, a new flag may sound safer.
Yang
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