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 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 <[email protected]> > > --- > > 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 */ > >
