[Virtio-fs] [PATCH 3/3] fuse: Add logic to upgrade a read-only mapping to read-write
piaojun
piaojun at huawei.com
Mon Aug 5 12:34:20 UTC 2019
Hi Vivek,
On 2019/8/5 20:25, Vivek Goyal wrote:
> On Sun, Aug 04, 2019 at 09:56:55AM +0800, piaojun wrote:
>> Hi Vivek,
>>
>> Shall we downgrade dax mapping from rw to read-only, if current user
>> open with O_RDONLY, and last user opened with O_RDWR? This will minimize
>> the access authority.
>>
>> I wonder if my concern is necessary. And if it will cause frequent dax
>> mapping?
>
> There is a chance that memory mapping will be reclaimed and setup as
> read-only again.
I got it, and thanks for your reply.
Thanks,
Jun
>
> I will not worry about this at this point of time.
>
> Thanks
> Vivek
>
>>
>> Thanks,
>> Jun
>>
>> On 2019/7/26 23:49, Vivek Goyal wrote:
>>> Do not always map a dax mapping read-write. There are use cases like
>>> executing a file where we want to map file read-only. virtio-fs dir on
>>> host might be backed by overlayfs. We don't want to open file read-write
>>> on overlayfs otherwise it will unnecessariliy be copied-up nullyifying
>>> the advantages of sharing page cache between vms for unmodified files.
>>>
>>> Hence, create a read-only mapping if user did not ask for writable mapping.
>>> Later upgrade it to read-write mapping when users requests it.
>>>
>>> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
>>> ---
>>> fs/fuse/file.c | 121 +++++++++++++++++++++++++++++++++++++----------
>>> fs/fuse/fuse_i.h | 3 ++
>>> 2 files changed, 98 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
>>> index a2c19e4a28b5..5277de7028a6 100644
>>> --- a/fs/fuse/file.c
>>> +++ b/fs/fuse/file.c
>>> @@ -265,7 +265,8 @@ static void dmap_add_to_free_pool(struct fuse_conn *fc,
>>>
>>> /* offset passed in should be aligned to FUSE_DAX_MEM_RANGE_SZ */
>>> static int fuse_setup_one_mapping(struct inode *inode, loff_t offset,
>>> - struct fuse_dax_mapping *dmap)
>>> + struct fuse_dax_mapping *dmap, bool writable,
>>> + bool upgrade)
>>> {
>>> struct fuse_conn *fc = get_fuse_conn(inode);
>>> struct fuse_inode *fi = get_fuse_inode(inode);
>>> @@ -283,7 +284,8 @@ static int fuse_setup_one_mapping(struct inode *inode, loff_t offset,
>>> inarg.moffset = dmap->window_offset;
>>> inarg.len = FUSE_DAX_MEM_RANGE_SZ;
>>> inarg.flags |= FUSE_SETUPMAPPING_FLAG_READ;
>>> - inarg.flags |= FUSE_SETUPMAPPING_FLAG_WRITE;
>>> + if (writable)
>>> + inarg.flags |= FUSE_SETUPMAPPING_FLAG_WRITE;
>>> args.in.h.opcode = FUSE_SETUPMAPPING;
>>> args.in.h.nodeid = fi->nodeid;
>>> args.in.numargs = 1;
>>> @@ -296,26 +298,30 @@ static int fuse_setup_one_mapping(struct inode *inode, loff_t offset,
>>> return err;
>>> }
>>>
>>> - pr_debug("fuse_setup_one_mapping() succeeded. offset=0x%llx err=%zd\n", offset, err);
>>> + pr_debug("fuse_setup_one_mapping() succeeded. offset=0x%llx writable=%d"
>>> + " err=%zd\n", offset, writable, err);
>>>
>>> - /*
>>> - * We don't take a refernce on inode. inode is valid right now and
>>> - * when inode is going away, cleanup logic should first cleanup
>>> - * dmap entries.
>>> - *
>>> - * TODO: Do we need to ensure that we are holding inode lock
>>> - * as well.
>>> - */
>>> - dmap->inode = inode;
>>> - dmap->start = offset;
>>> - dmap->end = offset + FUSE_DAX_MEM_RANGE_SZ - 1;
>>> - /* Protected by fi->i_dmap_sem */
>>> - fuse_dax_interval_tree_insert(dmap, &fi->dmap_tree);
>>> - fi->nr_dmaps++;
>>> - spin_lock(&fc->lock);
>>> - list_add_tail(&dmap->busy_list, &fc->busy_ranges);
>>> - fc->nr_busy_ranges++;
>>> - spin_unlock(&fc->lock);
>>> + dmap->writable = writable;
>>> + if (!upgrade) {
>>> + /*
>>> + * We don't take a refernce on inode. inode is valid right now
>>> + * and when inode is going away, cleanup logic should first
>>> + * cleanup dmap entries.
>>> + *
>>> + * TODO: Do we need to ensure that we are holding inode lock
>>> + * as well.
>>> + */
>>> + dmap->inode = inode;
>>> + dmap->start = offset;
>>> + dmap->end = offset + FUSE_DAX_MEM_RANGE_SZ - 1;
>>> + /* Protected by fi->i_dmap_sem */
>>> + fuse_dax_interval_tree_insert(dmap, &fi->dmap_tree);
>>> + fi->nr_dmaps++;
>>> + spin_lock(&fc->lock);
>>> + list_add_tail(&dmap->busy_list, &fc->busy_ranges);
>>> + fc->nr_busy_ranges++;
>>> + spin_unlock(&fc->lock);
>>> + }
>>> return 0;
>>> }
>>>
>>> @@ -1890,6 +1896,7 @@ static int iomap_begin_setup_new_mapping(struct inode *inode, loff_t pos,
>>> struct fuse_conn *fc = get_fuse_conn(inode);
>>> struct fuse_dax_mapping *dmap, *alloc_dmap = NULL;
>>> int ret;
>>> + bool writable = flags & IOMAP_WRITE;
>>>
>>> /* Can't do reclaim in fault path yet due to lock ordering.
>>> * Read path takes shared inode lock and that's not sufficient
>>> @@ -1930,10 +1937,11 @@ static int iomap_begin_setup_new_mapping(struct inode *inode, loff_t pos,
>>>
>>> /* Setup one mapping */
>>> ret = fuse_setup_one_mapping(inode,
>>> - ALIGN_DOWN(pos, FUSE_DAX_MEM_RANGE_SZ), alloc_dmap);
>>> + ALIGN_DOWN(pos, FUSE_DAX_MEM_RANGE_SZ),
>>> + alloc_dmap, writable, false);
>>> if (ret < 0) {
>>> printk("fuse_setup_one_mapping() failed. err=%d"
>>> - " pos=0x%llx\n", ret, pos);
>>> + " pos=0x%llx, writable=%d\n", ret, pos, writable);
>>> dmap_add_to_free_pool(fc, alloc_dmap);
>>> up_write(&fi->i_dmap_sem);
>>> return ret;
>>> @@ -1943,6 +1951,52 @@ static int iomap_begin_setup_new_mapping(struct inode *inode, loff_t pos,
>>> return 0;
>>> }
>>>
>>> +static int iomap_begin_upgrade_mapping(struct inode *inode, loff_t pos,
>>> + loff_t length, unsigned flags,
>>> + struct iomap *iomap)
>>> +{
>>> + struct fuse_inode *fi = get_fuse_inode(inode);
>>> + struct fuse_dax_mapping *dmap;
>>> + int ret;
>>> +
>>> + /*
>>> + * Take exclusive lock so that only one caller can try to setup
>>> + * mapping and others wait.
>>> + */
>>> + down_write(&fi->i_dmap_sem);
>>> + dmap = fuse_dax_interval_tree_iter_first(&fi->dmap_tree, pos, pos);
>>> +
>>> + /* We are holding either inode lock or i_mmap_sem, and that should
>>> + * ensure that dmap can't reclaimed or truncated and it should still
>>> + * be there in tree despite the fact we dropped and re-acquired the
>>> + * lock.
>>> + */
>>> + ret = -EIO;
>>> + if (WARN_ON(!dmap))
>>> + goto out_err;
>>> +
>>> + /* Maybe another thread already upgraded mapping while we were not
>>> + * holding lock.
>>> + */
>>> + if (dmap->writable)
>>> + goto out_fill_iomap;
>>> +
>>> + ret = fuse_setup_one_mapping(inode,
>>> + ALIGN_DOWN(pos, FUSE_DAX_MEM_RANGE_SZ),
>>> + dmap, true, true);
>>> + if (ret < 0) {
>>> + printk("fuse_setup_one_mapping() failed. err=%d pos=0x%llx\n",
>>> + ret, pos);
>>> + goto out_err;
>>> + }
>>> +
>>> +out_fill_iomap:
>>> + fuse_fill_iomap(inode, pos, length, iomap, dmap, flags);
>>> +out_err:
>>> + up_write(&fi->i_dmap_sem);
>>> + return ret;
>>> +}
>>> +
>>> /* This is just for DAX and the mapping is ephemeral, do not use it for other
>>> * purposes since there is no block device with a permanent mapping.
>>> */
>>> @@ -1952,6 +2006,7 @@ static int fuse_iomap_begin(struct inode *inode, loff_t pos, loff_t length,
>>> struct fuse_inode *fi = get_fuse_inode(inode);
>>> struct fuse_conn *fc = get_fuse_conn(inode);
>>> struct fuse_dax_mapping *dmap;
>>> + bool writable = flags & IOMAP_WRITE;
>>>
>>> /* We don't support FIEMAP */
>>> BUG_ON(flags & IOMAP_REPORT);
>>> @@ -1982,9 +2037,23 @@ static int fuse_iomap_begin(struct inode *inode, loff_t pos, loff_t length,
>>> dmap = fuse_dax_interval_tree_iter_first(&fi->dmap_tree, pos, pos);
>>>
>>> if (dmap) {
>>> - fuse_fill_iomap(inode, pos, length, iomap, dmap, flags);
>>> - up_read(&fi->i_dmap_sem);
>>> - return 0;
>>> + if (writable && !dmap->writable) {
>>> + /* Upgrade read-only mapping to read-write. This will
>>> + * require exclusive i_dmap_sem lock as we don't want
>>> + * two threads to be trying to this simultaneously
>>> + * for same dmap. So drop shared lock and acquire
>>> + * exclusive lock.
>>> + */
>>> + up_read(&fi->i_dmap_sem);
>>> + pr_debug("%s: Upgrading mapping at offset 0x%llx"
>>> + " length 0x%llx\n", __func__, pos, length);
>>> + return iomap_begin_upgrade_mapping(inode, pos, length,
>>> + flags, iomap);
>>> + } else {
>>> + fuse_fill_iomap(inode, pos, length, iomap, dmap, flags);
>>> + up_read(&fi->i_dmap_sem);
>>> + return 0;
>>> + }
>>> } else {
>>> up_read(&fi->i_dmap_sem);
>>> pr_debug("%s: no mapping at offset 0x%llx length 0x%llx\n",
>>> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
>>> index e899a06e29d7..34ca8b90a1e1 100644
>>> --- a/fs/fuse/fuse_i.h
>>> +++ b/fs/fuse/fuse_i.h
>>> @@ -134,6 +134,9 @@ struct fuse_dax_mapping {
>>>
>>> /** Length of mapping, in bytes */
>>> loff_t length;
>>> +
>>> + /* Is this mapping read-only or read-write */
>>> + bool writable;
>>> };
>>>
>>> /** FUSE inode */
>>>
> .
>
More information about the Virtio-fs
mailing list