> 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.
for EOPNOTSUPP?
glibc/sysdeps/unix/sysv/linux/i386/posix_fallocate.c
----------------
int
posix_fallocate (int fd, __off_t offset, __off_t len)
{
#ifdef __NR_fallocate
# ifndef __ASSUME_FALLOCATE
if (__builtin_expect (__have_fallocate >= 0, 1))
# endif
{
int res = __call_fallocate (fd, 0, offset, len);
if (! res)
return 0;
# ifndef __ASSUME_FALLOCATE
if (__builtin_expect (res == ENOSYS, 0))
__have_fallocate = -1;
else
# endif
if (res != EOPNOTSUPP)
return res;
}
#endif
return internal_fallocate (fd, offset, len);
}
--------------------------
But, ok, I'm now convinced this is needed. people strongly dislike to
receive SIGBUS. yes.
> 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;
do_fallocate has following file type check.
if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
return -ENODEV;
However, this implementation don't support dir allocation and/or punch hole.
ext4's ext4_punch_hole() has following additional check. Maybe we need similar
check.
if (!S_ISREG(inode->i_mode))
return -ENOTSUPP;
> + 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))
Seems incorrect.
fallocate(PUNCH, 0, very_big_number) should punch to a range of [0, end).
> + 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 (ret) {
> + if (ret == -ENOSPC)
> + goto undo;
> + else
> + goto unlock;
> + }
> + if (page) {
> + unlock_page(page);
> + page_cache_release(page);
> + }
> + index++;
> + }
> + if (!(mode & FALLOC_FL_KEEP_SIZE) && (index << PAGE_CACHE_SHIFT) > i_size)
> + i_size_write(inode, index << PAGE_CACHE_SHIFT);
Seems incorrect.
new i_size should be offset+len. our round-up is implementation detail
and don't have to expose
to userland.
> +
> + goto unlock;
> +
> +undo:
> + while (index > start) {
> + shmem_truncate_page(inode, index);
> + index--;
Hmmm...
seems too aggressive truncate if the file has pages before starting fallocate.
but I have no idea to make better undo. ;)
> + }
> +
> +unlock:
> + mutex_unlock(&inode->i_mutex);
> + return ret;
> +}
> +
> static int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
> {
> struct shmem_sb_info *sbinfo = SHMEM_SB(dentry->d_sb);
> @@ -2286,6 +2350,7 @@ static const struct file_operations shmem_file_operations = {
> .fsync = noop_fsync,
> .splice_read = shmem_file_splice_read,
> .splice_write = generic_file_splice_write,
> + .fallocate = shmem_fallocate,
> #endif
> };
--
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