I have a question. On Wed, 23 Nov 2011 16:53:30 +0800 Cong Wang <amwang@xxxxxxxxxx> wrote: > Systemd needs tmpfs to support fallocate [1], to be able > to safely use mmap(), regarding SIGBUS, on files on the > /dev/shm filesystem. The glibc fallback loop for -ENOSYS > on fallocate is just ugly. > > This patch adds fallocate support to tmpfs, and as we > already have shmem_truncate_range(), it is also easy to > add FALLOC_FL_PUNCH_HOLE support too. > > 1. http://lkml.org/lkml/2011/10/20/275 > > V2->V3: > a) Read i_size directly after holding i_mutex; > b) Call page_cache_release() too after shmem_getpage(); > c) Undo previous changes when -ENOSPC. > > Cc: Pekka Enberg <penberg@xxxxxxxxxx> > Cc: Christoph Hellwig <hch@xxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> > Cc: Lennart Poettering <lennart@xxxxxxxxxxxxxx> > Cc: Kay Sievers <kay.sievers@xxxxxxxx> > Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> > Signed-off-by: WANG Cong <amwang@xxxxxxxxxx> > > --- > mm/shmem.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 files changed, 65 insertions(+), 0 deletions(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index d672250..65f7a27 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -30,6 +30,7 @@ > #include <linux/mm.h> > #include <linux/export.h> > #include <linux/swap.h> > +#include <linux/falloc.h> > > static struct vfsmount *shm_mnt; > > @@ -1431,6 +1432,69 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos, > return error; > } > > +static void shmem_truncate_page(struct inode *inode, pgoff_t index) > +{ > + loff_t start = index << PAGE_CACHE_SHIFT; > + loff_t end = ((index + 1) << PAGE_CACHE_SHIFT) - 1; > + shmem_truncate_range(inode, start, end); > +} > + > +static long shmem_fallocate(struct file *file, int mode, > + loff_t offset, loff_t len) > +{ > + struct inode *inode = file->f_path.dentry->d_inode; > + pgoff_t start = offset >> PAGE_CACHE_SHIFT; > + pgoff_t end = DIV_ROUND_UP((offset + len), PAGE_CACHE_SIZE); > + pgoff_t index = start; > + loff_t i_size; > + struct page *page = NULL; > + int ret = 0; > + > + mutex_lock(&inode->i_mutex); > + i_size = inode->i_size; > + if (mode & FALLOC_FL_PUNCH_HOLE) { > + if (!(offset > i_size || (end << PAGE_CACHE_SHIFT) > i_size)) > + shmem_truncate_range(inode, offset, > + (end << PAGE_CACHE_SHIFT) - 1); > + goto unlock; > + } > + > + if (!(mode & FALLOC_FL_KEEP_SIZE)) { > + ret = inode_newsize_ok(inode, (offset + len)); > + if (ret) > + goto unlock; > + } > + > + while (index < end) { > + ret = shmem_getpage(inode, index, &page, SGP_WRITE, NULL); If the 'page' for index exists before this call, this will return the page without allocaton. Then, the page may not be zero-cleared. I think the page should be zero-cleared. But I'm not sure when we do zero-clear them. But I'm not sure how fallocate should work at error. Assume some block already exists before fallocate(), possible side-effect will be - the contents will be zero-cleared even if fallocate fails. - the contents will be deallocated in undo path if fallocate fails. ? maybe updates to man(2) fallocate will be appreciated... Anyway, don't you need zero-clear when you find an existing pages here ? Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>