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tmpfs: optimize clearing when writing
Nick proposed years ago that tmpfs should avoid clearing its pages where write will overwrite them with new data, as ramfs has long done. But I messed it up and just got bad data. Tried again recently, it works fine. Here's time output for writing 4GiB 16 times on this Core i5 laptop: before: real 0m21.169s user 0m0.028s sys 0m21.057s real 0m21.382s user 0m0.016s sys 0m21.289s real 0m21.311s user 0m0.020s sys 0m21.217s after: real 0m18.273s user 0m0.032s sys 0m18.165s real 0m18.354s user 0m0.020s sys 0m18.265s real 0m18.440s user 0m0.032s sys 0m18.337s ramfs: real 0m16.860s user 0m0.028s sys 0m16.765s real 0m17.382s user 0m0.040s sys 0m17.273s real 0m17.133s user 0m0.044s sys 0m17.021s Yes, I have done perf reports, but they need more explanation than they deserve: in summary, clear_page vanishes, its cache loading shifts into copy_user_generic_unrolled; shmem_getpage_gfp goes down, and surprisingly mark_page_accessed goes way up - I think because they are respectively where the cache gets to be reloaded after being purged by clear or copy. Suggested-by: Nick Piggin <npiggin@gmail.com> Signed-off-by: Hugh Dickins <hughd@google.com> Cc: Christoph Hellwig <hch@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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20
mm/shmem.c
20
mm/shmem.c
@ -1095,9 +1095,14 @@ repeat:
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shmem_recalc_inode(inode);
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spin_unlock(&info->lock);
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clear_highpage(page);
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flush_dcache_page(page);
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SetPageUptodate(page);
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/*
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* Let SGP_WRITE caller clear ends if write does not fill page
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*/
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if (sgp != SGP_WRITE) {
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clear_highpage(page);
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flush_dcache_page(page);
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SetPageUptodate(page);
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}
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if (sgp == SGP_DIRTY)
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set_page_dirty(page);
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}
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@ -1307,6 +1312,14 @@ shmem_write_end(struct file *file, struct address_space *mapping,
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if (pos + copied > inode->i_size)
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i_size_write(inode, pos + copied);
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if (!PageUptodate(page)) {
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if (copied < PAGE_CACHE_SIZE) {
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unsigned from = pos & (PAGE_CACHE_SIZE - 1);
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zero_user_segments(page, 0, from,
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from + copied, PAGE_CACHE_SIZE);
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}
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SetPageUptodate(page);
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}
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set_page_dirty(page);
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unlock_page(page);
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page_cache_release(page);
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@ -1768,6 +1781,7 @@ static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *s
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kaddr = kmap_atomic(page);
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memcpy(kaddr, symname, len);
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kunmap_atomic(kaddr);
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SetPageUptodate(page);
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set_page_dirty(page);
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unlock_page(page);
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page_cache_release(page);
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