[RFC] erofs-utils:code for detecting and tracking holes in uncompressed sparse files.
Gao Xiang
gaoxiang25 at huawei.com
Mon Dec 9 18:18:17 AEDT 2019
Hi Pratik,
On Mon, Dec 09, 2019 at 12:34:50PM +0530, Pratik Shinde wrote:
> Hello Gao,
>
> Did you get any chance to look at this in detail.
I looked into your implementation weeks before.
As my reply in the previous email, did you see it and could you check it out?
Kernel emails are not top-posting so you could scroll down to the end.
> > "variable-sized inode" isn't a problem here, which can be handled
> > similar to the case of "compress indexes".
> >
> > Probably no need to write linked list to the disk but generate linked list
> > in memory when writing data on the fly, and then transfer to a
> > variable-sized
> > extent array at the time of writing inode metadata (The order is firstly
> > data
> > and then metadata in erofs-utils so it looks practical.)
Thanks,
Gao Xiang
>
> --Pratik.
>
> On Wed, Dec 4, 2019, 7:52 AM Gao Xiang <gaoxiang25 at huawei.com> wrote:
>
> > Hi Pratik,
> >
> > I'll give detailed words this weekend if you have more questions
> > since I'm busying in other stupid intra-company stuffs now...
> >
> > On Tue, Dec 03, 2019 at 07:32:50PM +0530, Pratik Shinde wrote:
> > > NOTE: The patch is not fully complete yet, with this patch I just want to
> > > present rough idea of what I am trying to achieve.
> > >
> > > The patch does following :
> > > 1) Detect holes (of size EROFS_BLKSIZ) in uncompressed files.
> > > 2) Keep track of holes per file.
> > >
> > > In-order to track holes, I used an array of size = (file_size /
> > blocksize)
> > > The array basically tracks number of holes before a particular logical
> > file block.
> > > e.g blks[i] = 10 meaning ith block has 10 holes before it.
> > > If a particular block is a hole we set the index to '-1'.
> > >
> > > how read logic will change:
> > > 1) currently we simply map read offset to a fs block.
> > > 2) with holes in place the calculation of block number would be:
> > >
> > > blkno = start_block + (offset >> block_size_shift) - (number of
> > > holes before
> > block in which offset falls)
> > >
> > > 3) If a read offset falls inside a hole (which can be found using above
> > array). We
> > > fill the user buffer with '\0' on the fly.
> > >
> > > through this,block no. lookup would still be performed in constant time.
> > >
> > > The biggest problem with this approach is - we have to store the hole
> > tracking
> > > array for every file to the disk.Which doesn't seems to be practical.we
> > can use a linkedlist,
> > > but that will make size of inode variable.
> >
> > "variable-sized inode" isn't a problem here, which can be handled
> > similar to the case of "compress indexes".
> >
> > Probably no need to write linked list to the disk but generate linked list
> > in memory when writing data on the fly, and then transfer to a
> > variable-sized
> > extent array at the time of writing inode metadata (The order is firstly
> > data
> > and then metadata in erofs-utils so it looks practical.)
> >
> > Thanks,
> > Gao Xiang
> >
> > >
> > > Signed-off-by: Pratik Shinde <pratikshinde320 at gmail.com>
> > > ---
> > > lib/inode.c | 67
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> > > 1 file changed, 66 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/lib/inode.c b/lib/inode.c
> > > index 0e19b11..af31949 100644
> > > --- a/lib/inode.c
> > > +++ b/lib/inode.c
> > > @@ -38,6 +38,61 @@ static unsigned char erofs_type_by_mode[S_IFMT >>
> > S_SHIFT] = {
> > >
> > > struct list_head inode_hashtable[NR_INODE_HASHTABLE];
> > >
> > > +
> > > +#define IS_HOLE(start, end) (roundup(start, EROFS_BLKSIZ) == start &&
> > \
> > > + roundup(end, EROFS_BLKSIZ) == end && \
> > > + (end - start) % EROFS_BLKSIZ == 0)
> > > +#define HOLE_BLK -1
> > > +unsigned int erofs_detect_holes(struct erofs_inode *inode, int *blks)
> > > +{
> > > + int i, fd, st, en;
> > > + unsigned int nblocks;
> > > + erofs_off_t data, hole, len;
> > > +
> > > + nblocks = inode->i_size / EROFS_BLKSIZ;
> > > + for (i = 0; i < nblocks; i++)
> > > + blks[i] = 0;
> > > + fd = open(inode->i_srcpath, O_RDONLY);
> > > + if (fd < 0) {
> > > + return -errno;
> > > + }
> > > + len = lseek(fd, 0, SEEK_END);
> > > + if (lseek(fd, 0, SEEK_SET) == -1)
> > > + return -errno;
> > > + data = 0;
> > > + while (data < len) {
> > > + hole = lseek(fd, data, SEEK_HOLE);
> > > + if (hole == len)
> > > + break;
> > > + data = lseek(fd, hole, SEEK_DATA);
> > > + if (data < 0 || hole > data) {
> > > + return -EINVAL;
> > > + }
> > > + if (IS_HOLE(hole, data)) {
> > > + st = hole >> S_SHIFT;
> > > + en = data >> S_SHIFT;
> > > + nblocks -= (en - st);
> > > + for (i = st; i < en; i++)
> > > + blks[i] = HOLE_BLK;
> > > + }
> > > + }
> > > + return nblocks;
> > > +}
> > > +
> > > +int erofs_fill_holedata(int *blks, unsigned int nblocks) {
> > > + int i, nholes = 0;
> > > + for (i = 0; i < nblocks; i++) {
> > > + if (blks[i] == -1)
> > > + nholes++;
> > > + else {
> > > + blks[i] = nholes;
> > > + if (nholes >= (i + 1))
> > > + return -EINVAL;
> > > + }
> > > + }
> > > + return 0;
> > > +}
> > > +
> > > void erofs_inode_manager_init(void)
> > > {
> > > unsigned int i;
> > > @@ -305,6 +360,7 @@ static bool erofs_file_is_compressible(struct
> > erofs_inode *inode)
> > > int erofs_write_file(struct erofs_inode *inode)
> > > {
> > > unsigned int nblocks, i;
> > > + int *blks;
> > > int ret, fd;
> > >
> > > if (!inode->i_size) {
> > > @@ -322,7 +378,13 @@ int erofs_write_file(struct erofs_inode *inode)
> > > /* fallback to all data uncompressed */
> > > inode->datalayout = EROFS_INODE_FLAT_INLINE;
> > > nblocks = inode->i_size / EROFS_BLKSIZ;
> > > -
> > > + blks = malloc(sizeof(int) * nblocks);
> > > + nblocks = erofs_detect_holes(inode, blks);
> > > + if (nblocks < 0)
> > > + return nblocks;
> > > + if ((ret = erofs_fill_holedata(blks, nblocks)) != 0) {
> > > + return ret;
> > > + }
> > > ret = __allocate_inode_bh_data(inode, nblocks);
> > > if (ret)
> > > return ret;
> > > @@ -332,6 +394,8 @@ int erofs_write_file(struct erofs_inode *inode)
> > > return -errno;
> > >
> > > for (i = 0; i < nblocks; ++i) {
> > > + if (blks[i] == HOLE_BLK)
> > > + continue;
> > > char buf[EROFS_BLKSIZ];
> > >
> > > ret = read(fd, buf, EROFS_BLKSIZ);
> > > @@ -962,3 +1026,4 @@ struct erofs_inode
> > *erofs_mkfs_build_tree_from_path(struct erofs_inode *parent,
> > > return erofs_mkfs_build_tree(inode);
> > > }
> > >
> > > +
> > > --
> > > 2.9.3
> > >
> >
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