On Mon, 5 Mar 2012, Allison Henderson wrote:
> On 03/02/2012 03:26 AM, Lukas Czerner wrote:
> > This commit rewrites ext4 punch hole implementation to use
> > ext4_ext_remove_space() instead of its home gown way of doing this via
> > ext4_ext_map_blocks(). There are several reasons for changing this.
> >
> > Firstly it is quite non obvious that punching hole needs to
> > ext4_ext_map_blocks() to punch a hole, especially given that this
> > function should map blocks, not unmap it. It also required a lot of new
> > code in ext4_ext_map_blocks().
> >
> > Secondly the design of it is not very effective. The reason is that we
> > are trying to punch out blocks in ext4_ext_punch_hole() in opposite
> > direction than in ext4_ext_rm_leaf() which causes the ext4_ext_rm_leaf()
> > to iterate through the whole tree from the end to the start to find the
> > requested extent for every extent we are going to punch out.
> >
> > And finally the current implementation does not use the existing code,
> > but bring a lot of new code, which is IMO unnecessary since there
> > already is some infrastructure we can use. Specifically
> > ext4_ext_remove_space().
> >
> > This commit changes ext4_ext_remove_space() to accept 'end' parameter so
> > we can not only truncate to the end of file, but also remove the space
> > in the middle of the file (punch a hole). Moreover, because the last
> > block to punch out, might be in the middle of the extent, we have to
> > split the extent at 'end + 1' so ext4_ext_rm_leaf() can easily either
> > remove the whole fist part of split extent, or change its size.
> >
> > ext4_ext_remove_space() is then used to actually remove the space
> > (extents) from within the hole, instead of ext4_ext_map_blocks().
> >
> > Note that this also fix the issue with punch hole, where we would forget
> > to remove empty index blocks from the extent tree, resulting in double
> > free block error and file system corruption. This is simply because we
> > now use different code path, where this problem does not exist.
> >
> > This has been tested with fsx running for several days and xfstests,
> > plus xfstest #251 with '-o discard' run on the loop image (which
> > converts discard requestes into punch hole to the backing file). All of
> > it on 1K and 4K file system block size.
> >
> > Signed-off-by: Lukas Czerner<lczerner@xxxxxxxxxx>
>
> Hi Lukas,
>
> Thank you for taking the time to go back over the punch hole code, I do like
> the new set up, and I think it looks cleaner :). There are some things though
> that are in the current solution that I do not see in this new solution, but I
> am hoping that maybe we dont need them since you seem to be getting through
> the tests ok. But I thought that I should point out they are there just in
> case something happens, we are aware of them. Also xfstests 255 and 256 are
> good punch hole tests to run through, if you havent tried them out yet.
Hi Allison,
thank you for the review. I did run a lot of xfstests including the two
you mentioned.
>
> Also, there's another unmerged patch out there that I will need to rebase on
> top of this one. It's not a big change, but there is one thing in this patch
> that will need to change to make it work. I go over that too in the comments
> below.
I'll try to see what it is all about, but since I just realized that
indeed we do not hold imutex, then we should definitely handle end >
isize case. We would probably need a xfstest for that :). Thanks!
>
>
> > ---
> > fs/ext4/extents.c | 174
> > ++++++++++++++++++++++++++++-------------------------
> > 1 files changed, 92 insertions(+), 82 deletions(-)
> >
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 74f23c2..04dd188 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -44,6 +44,14 @@
> >
> > #include<trace/events/ext4.h>
> >
> > +/*
> > + * used by extent splitting.
> > + */
> > +#define EXT4_EXT_MAY_ZEROOUT 0x1 /* safe to zeroout if split fails \
> > + due to ENOSPC */
> > +#define EXT4_EXT_MARK_UNINIT1 0x2 /* mark first half uninitialized
> > */
> > +#define EXT4_EXT_MARK_UNINIT2 0x4 /* mark second half uninitialized
> > */
> > +
> > static int ext4_split_extent(handle_t *handle,
> > struct inode *inode,
> > struct ext4_ext_path *path,
> > @@ -51,6 +59,13 @@ static int ext4_split_extent(handle_t *handle,
> > int split_flag,
> > int flags);
> >
> > +static int ext4_split_extent_at(handle_t *handle,
> > + struct inode *inode,
> > + struct ext4_ext_path *path,
> > + ext4_lblk_t split,
> > + int split_flag,
> > + int flags);
> > +
> > static int ext4_ext_truncate_extend_restart(handle_t *handle,
> > struct inode *inode,
> > int needed)
> > @@ -2308,7 +2323,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode
> > *inode,
> > struct ext4_extent *ex;
> >
> > /* the header must be checked already in ext4_ext_remove_space() */
> > - ext_debug("truncate since %u in leaf\n", start);
> > + ext_debug("truncate since %u in leaf to %u\n", start, end);
> > if (!path[depth].p_hdr)
> > path[depth].p_hdr = ext_block_hdr(path[depth].p_bh);
> > eh = path[depth].p_hdr;
> > @@ -2343,7 +2358,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode
> > *inode,
> > ext_debug(" border %u:%u\n", a, b);
> >
> > /* If this extent is beyond the end of the hole, skip it */
> > - if (end<= ex_ee_block) {
> > + if (end< ex_ee_block) {
> > ex--;
> > ex_ee_block = le32_to_cpu(ex->ee_block);
> > ex_ee_len = ext4_ext_get_actual_len(ex);
> > @@ -2482,16 +2497,18 @@ ext4_ext_more_to_rm(struct ext4_ext_path *path)
> > return 1;
> > }
> >
> > -static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start)
> > +static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
> > + ext4_lblk_t end)
> > {
> > struct super_block *sb = inode->i_sb;
> > int depth = ext_depth(inode);
> > struct ext4_ext_path *path;
> > ext4_fsblk_t partial_cluster = 0;
> > handle_t *handle;
> > + ext4_lblk_t size;
> > int i, err;
> >
> > - ext_debug("truncate since %u\n", start);
> > + ext_debug("truncate since %u to %u\n", start, end);
> >
> > /* probably first extent we're gonna free will be last in block */
> > handle = ext4_journal_start(inode, depth + 1);
> > @@ -2503,6 +2520,64 @@ again:
> >
> > trace_ext4_ext_remove_space(inode, start, depth);
> >
>
> So in this snippet below it looks like you do the splitting only if the hole
> is contained with in i_size. I think though that we need to allow punching
> beyond i_size. A while ago Hugh found this bug (this email: "punch-hole
> should go beyond i_size" sent around 1/11/2012). So, I sent out a patch for
> it ("[PATCH 0/3] ext4: punch hole beyond i_size" sent around 1/14/2012 ), but
> I think it just got lost in the traffic, because I dont think it got merged.
> I've been meaning to resend but have just been tied up. I try as much as I
> can to keep up with ext4 on my own time, but I do not get much time allotted
> to it anymore :( . The set actually changes code up in ext4_ext_punch_hole,
> and I dont think your patch touches the same code, so I should be able to
> rebase it on top of your patch fairly easily.
> I will send an update based on your new set, but it looks like the line below
> will need to change too. In this case, I think what what you're meaning to do
> is split extents if we are punching holes (right?). Maybe instead of using
> i_size, we could just check "if (end != EXT_MAX_BLOCKS - 1)", since truncate
> will always use "EXT_MAX_BLOCKS - 1"
Yes, in that case "if (end != EXT_MAX_BLOCKS - 1)" would probably make
more sense. I'll try to come up with the test case to trigger the
problem. Thanks for pointing it out.
>
> > + size = (inode->i_size + sb->s_blocksize - 1)
> > + >> EXT4_BLOCK_SIZE_BITS(sb);
> > +
> > + /*
> > + * Check if we are removing extents inside the extent tree. If that
> > + * is the case, we are going to punch a hole inside the extent tree
> > + * so we have to check whether we need to split the extent covering
> > + * the last block to remove so we can easily remove the part of it
> > + * in ext4_ext_rm_leaf().
> > + */
> > + if (size> end) {
> > + struct ext4_extent *ex;
> > + ext4_lblk_t ee_block;
> > +
> > + /* find extent for this block */
> > + path = ext4_ext_find_extent(inode, end, NULL);
> > + if (IS_ERR(path)) {
> > + ext4_journal_stop(handle);
> > + return PTR_ERR(path);
> > + }
> > + depth = ext_depth(inode);
> > + ex = path[depth].p_ext;
> > + if (!ex)
> > + goto cont;
> > +
> > + ee_block = le32_to_cpu(ex->ee_block);
> > +
>
> Some comments on the split logic here: When my earlier versions of the punch
> hole patches were getting reviewed, I remember one of the things that came up
> was that the extents need to be marked uninitialized before being removed. In
> this code, it looks like you mark them only if they are already uninitialized,
> and at the end of the hole.
>
> The reason this changes the split logic is because initialized extents can be
> larger than uninitialized extents. So you cant just mark it uninitialized
> with out splitting. Even if it's in the middle of the hole, you may have to
> split it anyway to make it fit in an uninitialized extent.
>
> In the previous solution, this worked because the split logic, being inside
> map_blocks, was in the body of the loop (called from ext4_ext_punch_hole), and
> the search started from the beginning each time we looked up an extent. In
> this case though, it might get tricky because this loop is trying to walk the
> extent tree.
>
> As I recall I think the reason we were marking them uninitialized was because
> we wanted them to read back zeros while the punch hole was in progress. Punch
> hole does not take i_mutex in fallocate, and since I was moved, I havnt been
> able to get much time in on extent locks. I suppose i_mutex would be a quick
> fix, but I realize Ted wanted to avoid further use of i_mutex since we really
> shouldn't be using it at all. Maybe we can get some more folks in on this
> discussion here, because if we really dont need them to be uninitialized, this
> solution is really simple and clean. :)
Hmm, I have not realized that we actually need to read zeroes from the
punched out extents before they are actually punched out. I need to take
a closer look at this but, in this case we'll have to use imutex anyway
to make marking the extents as uninitialized atomic operation. If it is
the case, then I am not sure what we gain here as opposed to just remove
the extents under imutex. But I guess I need to look at this problem
more closely. Thanks for pointing it out.
Anyway, the extent locks might be a help here, maybe we can cooperate on
this to come up with something sooner rather than later ?
Thanks for review Allison.
-Lukas
>
> > + /*
> > + * See if the last block is inside the extent, if so split
> > + * the extent at 'end' block so we can easily remove the
> > + * tail of the first part of the split extent in
> > + * ext4_ext_rm_leaf().
> > + */
> > + if (end>= ee_block&&
> > + end< ee_block + ext4_ext_get_actual_len(ex) - 1) {
> > + int split_flag = 0;
> > +
> > + if (ext4_ext_is_uninitialized(ex))
> > + split_flag = EXT4_EXT_MARK_UNINIT1 |
> > + EXT4_EXT_MARK_UNINIT2;
> > +
> > + /*
> > + * Split the extent in two so that 'end' is the last
> > + * block in the first new extent
> > + */
> > + err = ext4_split_extent_at(handle, inode, path,
> > + end + 1, split_flag,
> > + EXT4_GET_BLOCKS_PRE_IO |
> > +
> > EXT4_GET_BLOCKS_PUNCH_OUT_EXT);
> > +
> > + if (err< 0)
> > + goto out;
> > + }
> > + ext4_ext_drop_refs(path);
> > + kfree(path);
> > + }
> > +cont:
> > +
> > /*
> > * We start scanning from right side, freeing all the blocks
> > * after i_size and walking into the tree depth-wise.
> > @@ -2515,6 +2590,7 @@ again:
> > }
> > path[0].p_depth = depth;
> > path[0].p_hdr = ext_inode_hdr(inode);
> > +
> > if (ext4_ext_check(inode, path[0].p_hdr, depth)) {
> > err = -EIO;
> > goto out;
> > @@ -2526,7 +2602,7 @@ again:
> > /* this is leaf block */
> > err = ext4_ext_rm_leaf(handle, inode, path,
> > &partial_cluster, start,
> > - EXT_MAX_BLOCKS - 1);
> > + end);
> > /* root level has p_bh == NULL, brelse() eats this */
> > brelse(path[i].p_bh);
> > path[i].p_bh = NULL;
> > @@ -2709,14 +2785,6 @@ static int ext4_ext_zeroout(struct inode *inode,
> > struct ext4_extent *ex)
> > }
> >
> > /*
> > - * used by extent splitting.
> > - */
> > -#define EXT4_EXT_MAY_ZEROOUT 0x1 /* safe to zeroout if split fails \
> > - due to ENOSPC */
> > -#define EXT4_EXT_MARK_UNINIT1 0x2 /* mark first half uninitialized
> > */
> > -#define EXT4_EXT_MARK_UNINIT2 0x4 /* mark second half uninitialized
> > */
> > -
> > -/*
> > * ext4_split_extent_at() splits an extent at given block.
> > *
> > * @handle: the journal handle
> > @@ -4228,7 +4296,7 @@ void ext4_ext_truncate(struct inode *inode)
> >
> > last_block = (inode->i_size + sb->s_blocksize - 1)
> > >> EXT4_BLOCK_SIZE_BITS(sb);
> > - err = ext4_ext_remove_space(inode, last_block);
> > + err = ext4_ext_remove_space(inode, last_block, EXT_MAX_BLOCKS - 1);
> >
> > /* In a multi-transaction truncate, we only make the final
> > * transaction synchronous.
> > @@ -4705,14 +4773,12 @@ int ext4_ext_punch_hole(struct file *file, loff_t
> > offset, loff_t length)
> > {
> > struct inode *inode = file->f_path.dentry->d_inode;
> > struct super_block *sb = inode->i_sb;
> > - struct ext4_ext_cache cache_ex;
> > - ext4_lblk_t first_block, last_block, num_blocks, iblock, max_blocks;
> > + ext4_lblk_t first_block, stop_block;
> > struct address_space *mapping = inode->i_mapping;
> > - struct ext4_map_blocks map;
> > handle_t *handle;
> > loff_t first_page, last_page, page_len;
> > loff_t first_page_offset, last_page_offset;
> > - int ret, credits, blocks_released, err = 0;
> > + int credits, err = 0;
> >
>
> This thing right here disappears with that other unmerged patch. It looks
> like you hop over it though, so hopefully it wont be a problem
> > /* No need to punch hole beyond i_size */
> > if (offset>= inode->i_size)
> > @@ -4728,10 +4794,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t
> > offset, loff_t length)
> > offset;
> > }
> >
> > - first_block = (offset + sb->s_blocksize - 1)>>
> > - EXT4_BLOCK_SIZE_BITS(sb);
> > - last_block = (offset + length)>> EXT4_BLOCK_SIZE_BITS(sb);
> > -
> > first_page = (offset + PAGE_CACHE_SIZE - 1)>> PAGE_CACHE_SHIFT;
> > last_page = (offset + length)>> PAGE_CACHE_SHIFT;
> >
> > @@ -4810,7 +4872,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t
> > offset, loff_t length)
> > }
> > }
> >
> > -
> > /*
> > * If i_size is contained in the last page, we need to
> > * unmap and zero the partial page after i_size
> > @@ -4830,73 +4891,22 @@ int ext4_ext_punch_hole(struct file *file, loff_t
> > offset, loff_t length)
> > }
> > }
> >
> > + first_block = (offset + sb->s_blocksize - 1)>>
> > + EXT4_BLOCK_SIZE_BITS(sb);
> > + stop_block = (offset + length)>> EXT4_BLOCK_SIZE_BITS(sb);
> > +
> > /* If there are no blocks to remove, return now */
> > - if (first_block>= last_block)
> > + if (first_block>= stop_block)
> > goto out;
> >
> > down_write(&EXT4_I(inode)->i_data_sem);
> > ext4_ext_invalidate_cache(inode);
> > ext4_discard_preallocations(inode);
> >
> > - /*
> > - * Loop over all the blocks and identify blocks
> > - * that need to be punched out
> > - */
> > - iblock = first_block;
> > - blocks_released = 0;
> > - while (iblock< last_block) {
> > - max_blocks = last_block - iblock;
> > - num_blocks = 1;
> > - memset(&map, 0, sizeof(map));
> > - map.m_lblk = iblock;
> > - map.m_len = max_blocks;
> > - ret = ext4_ext_map_blocks(handle, inode,&map,
> > - EXT4_GET_BLOCKS_PUNCH_OUT_EXT);
> > -
> > - if (ret> 0) {
> > - blocks_released += ret;
> > - num_blocks = ret;
> > - } else if (ret == 0) {
> > - /*
> > - * If map blocks could not find the block,
> > - * then it is in a hole. If the hole was
> > - * not already cached, then map blocks should
> > - * put it in the cache. So we can get the hole
> > - * out of the cache
> > - */
> > - memset(&cache_ex, 0, sizeof(cache_ex));
> > - if ((ext4_ext_check_cache(inode, iblock,&cache_ex))&&
> > - !cache_ex.ec_start) {
> > + err = ext4_ext_remove_space(inode, first_block, stop_block - 1);
> >
> > - /* The hole is cached */
> > - num_blocks = cache_ex.ec_block +
> > - cache_ex.ec_len - iblock;
> > -
> > - } else {
> > - /* The block could not be identified */
> > - err = -EIO;
> > - break;
> > - }
> > - } else {
> > - /* Map blocks error */
> > - err = ret;
> > - break;
> > - }
> > -
> > - if (num_blocks == 0) {
> > - /* This condition should never happen */
> > - ext_debug("Block lookup failed");
> > - err = -EIO;
> > - break;
> > - }
> > -
> > - iblock += num_blocks;
> > - }
> > -
> > - if (blocks_released> 0) {
> > - ext4_ext_invalidate_cache(inode);
> > - ext4_discard_preallocations(inode);
> > - }
> > + ext4_ext_invalidate_cache(inode);
> > + ext4_discard_preallocations(inode);
> >
> > if (IS_SYNC(inode))
> > ext4_handle_sync(handle);
>
> That's all my comments for now, the rest of it looks really nice. Thank you
> for taking the time to go through it!
>
> Allison Henderson
>
>
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