On Tue, Jul 09, 2024 at 12:10:21PM -0700, Wengang Wang wrote:
> segments are the smallest unit to defragment.
>
> A segment
> 1. Can't exceed size limit
> 2. contains some extents
> 3. the contained extents can't be "unwritten"
> 4. the contained extents must be contigous in file blocks
>
> Signed-off-by: Wengang Wang <wen.gang.wang@xxxxxxxxxx>
> ---
> spaceman/defrag.c | 204 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 204 insertions(+)
>
> diff --git a/spaceman/defrag.c b/spaceman/defrag.c
> index c9732984..175cf461 100644
> --- a/spaceman/defrag.c
> +++ b/spaceman/defrag.c
> @@ -14,6 +14,32 @@
> #include "space.h"
> #include "input.h"
>
> +#define MAPSIZE 512
> +/* used to fetch bmap */
> +struct getbmapx g_mapx[MAPSIZE];
> +/* current offset of the file in units of 512 bytes, used to fetch bmap */
> +static long long g_offset = 0;
> +/* index to indentify next extent, used to get next extent */
> +static int g_ext_next_idx = -1;
Please do not prefix global variables with "g_". This is not
useful, and simply makes the code hard to read.
That said, it is much better to pass these as function parameters so
they are specific to the mapping context and so are inherently
thread safe.
> +/*
> + * segment, the smallest unit to defrag
> + * it includes some contiguous extents.
> + * no holes included,
> + * no unwritten extents included
> + * the size is limited by g_segment_size_lmt
> + */
I have no idea what this comment is trying to tell me.
> +struct defrag_segment {
> + /* segment offset in units of 512 bytes */
> + long long ds_offset;
> + /* length of segment in units of 512 bytes */
> + long long ds_length;
> + /* number of extents in this segment */
> + int ds_nr;
> + /* flag indicating if segment contains shared blocks */
> + bool ds_shared;
> +};
> +
> /* defrag segment size limit in units of 512 bytes */
> #define MIN_SEGMENT_SIZE_LIMIT 8192 /* 4MiB */
> #define DEFAULT_SEGMENT_SIZE_LIMIT 32768 /* 16MiB */
> @@ -78,6 +104,165 @@ defrag_check_file(char *path)
> return true;
> }
>
> +/*
> + * get next extent in the file.
> + * Note: next call will get the same extent unless move_next_extent() is called.
> + * returns:
> + * -1: error happened.
> + * 0: extent returned
> + * 1: no more extent left
> + */
> +static int
> +defrag_get_next_extent(int fd, struct getbmapx *map_out)
> +{
> + int err = 0, i;
> +
> + /* when no extents are cached in g_mapx, fetch from kernel */
> + if (g_ext_next_idx == -1) {
> + g_mapx[0].bmv_offset = g_offset;
> + g_mapx[0].bmv_length = -1LL;
> + g_mapx[0].bmv_count = MAPSIZE;
> + g_mapx[0].bmv_iflags = BMV_IF_NO_HOLES | BMV_IF_PREALLOC;
> + err = ioctl(fd, XFS_IOC_GETBMAPX, g_mapx);
> + if (err == -1) {
> + perror("XFS_IOC_GETBMAPX failed");
> + goto out;
> + }
> + /* for stats */
> + g_ext_stats.nr_ext_total += g_mapx[0].bmv_entries;
> +
> + /* no more extents */
> + if (g_mapx[0].bmv_entries == 0) {
> + err = 1;
> + goto out;
> + }
> +
> + /* for stats */
> + for (i = 1; i <= g_mapx[0].bmv_entries; i++) {
> + if (g_mapx[i].bmv_oflags & BMV_OF_PREALLOC)
> + g_ext_stats.nr_ext_unwritten++;
> + if (g_mapx[i].bmv_oflags & BMV_OF_SHARED)
> + g_ext_stats.nr_ext_shared++;
> + }
> +
> + g_ext_next_idx = 1;
> + g_offset = g_mapx[g_mapx[0].bmv_entries].bmv_offset +
> + g_mapx[g_mapx[0].bmv_entries].bmv_length;
> + }
> +
> + map_out->bmv_offset = g_mapx[g_ext_next_idx].bmv_offset;
> + map_out->bmv_length = g_mapx[g_ext_next_idx].bmv_length;
> + map_out->bmv_oflags = g_mapx[g_ext_next_idx].bmv_oflags;
> +out:
> + return err;
> +}
Ok, so the global variables are just a bmap cache. That's a problem,
because this cache is stale the moment XFS_IOC_GETBMAPX returns to
userspace. Iterating it to decide exactly waht to do next will
race with ongoing file modifications and so it's not going to be
accurate....
> +
> +/*
> + * move to next extent
> + */
> +static void
> +defrag_move_next_extent()
> +{
> + if (g_ext_next_idx == g_mapx[0].bmv_entries)
> + g_ext_next_idx = -1;
> + else
> + g_ext_next_idx += 1;
> +}
> +
> +/*
> + * check if the given extent is a defrag target.
> + * no need to check for holes as we are using BMV_IF_NO_HOLES
> + */
> +static bool
> +defrag_is_target(struct getbmapx *mapx)
> +{
> + /* unwritten */
> + if (mapx->bmv_oflags & BMV_OF_PREALLOC)
> + return false;
> + return mapx->bmv_length < g_segment_size_lmt;
> +}
> +
> +static bool
> +defrag_is_extent_shared(struct getbmapx *mapx)
> +{
> + return !!(mapx->bmv_oflags & BMV_OF_SHARED);
> +}
> +
> +/*
> + * get next segment to defragment.
> + * returns:
> + * -1 error happened.
> + * 0 segment returned.
> + * 1 no more segments to return
> + */
> +static int
> +defrag_get_next_segment(int fd, struct defrag_segment *out)
> +{
> + struct getbmapx mapx;
> + int ret;
> +
> + out->ds_offset = 0;
> + out->ds_length = 0;
> + out->ds_nr = 0;
> + out->ds_shared = false;
out->ds_nr is never set to anything but zero in this patch.
> +
> + do {
> + ret = defrag_get_next_extent(fd, &mapx);
> + if (ret != 0) {
> + /*
> + * no more extetns, return current segment if its not
Typos everywhere.
> + * empty
> + */
> + if (ret == 1 && out->ds_nr > 0)
> + ret = 0;
> + /* otherwise, error heppened, stop */
> + break;
> + }
> +
> + /*
> + * If the extent is not a defrag target, skip it.
> + * go to next extent if the segment is empty;
> + * otherwise return the segment.
> + */
> + if (!defrag_is_target(&mapx)) {
> + defrag_move_next_extent();
> + if (out->ds_nr == 0)
> + continue;
> + else
> + break;
> + }
> +
> + /* check for segment size limitation */
> + if (out->ds_length + mapx.bmv_length > g_segment_size_lmt)
> + break;
> +
> + /* the segment is empty now, add this extent to it for sure */
> + if (out->ds_nr == 0) {
> + out->ds_offset = mapx.bmv_offset;
> + goto add_ext;
> + }
So this is essentially a filter for the getbmapx output that strips
away unwritten extents and anything outside/larger than the target
range.
> +
> + /*
> + * the segment is not empty, check for hole since the last exent
> + * if a hole exist before this extent, this extent can't be
> + * added to the segment. return the segment
> + */
> + if (out->ds_offset + out->ds_length != mapx.bmv_offset)
> + break;
> +
> +add_ext:
Why do you need a goto for this logic?
/*
* the segment is not empty, check for hole since the last exent
* if a hole exist before this extent, this extent can't be
* added to the segment. return the segment
*/
if (out->ds_nr) {
if (out->ds_offset + out->ds_length != mapx.bmv_offset)
break;
} else {
out->ds_offset = mapx.bmv_offset;
}
> + if (defrag_is_extent_shared(&mapx))
> + out->ds_shared = true;
> +
> + out->ds_length += mapx.bmv_length;
> + out->ds_nr += 1;
> + defrag_move_next_extent();
> +
> + } while (true);
> +
> + return ret;
> +}
> +
> /*
> * defragment a file
> * return 0 if successfully done, 1 otherwise
> @@ -92,6 +277,9 @@ defrag_xfs_defrag(char *file_path) {
> struct fsxattr fsx;
> int ret = 0;
>
> + g_offset = 0;
> + g_ext_next_idx = -1;
> +
> fsx.fsx_nextents = 0;
> memset(&g_ext_stats, 0, sizeof(g_ext_stats));
>
> @@ -119,6 +307,22 @@ defrag_xfs_defrag(char *file_path) {
> ret = 1;
> goto out;
> }
> +
> + do {
> + struct defrag_segment segment;
> +
> + ret = defrag_get_next_segment(defrag_fd, &segment);
> + /* no more segments, we are done */
> + if (ret == 1) {
> + ret = 0;
> + break;
> + }
> + /* error happened when reading bmap, stop here */
> + if (ret == -1) {
> + ret = 1;
> + break;
> + }
ternary return values are nasty. Return a negative errno when a
an error occurs, and -ENODATA when there are no more segments.
Then you have
if (ret < 0) {
if (ret == -ENODATA)
exit_value = 0;
else
exit_value = 1;
break;
}
> + } while (true);
Not a fan of do {} while(true) loops.
WIth the above error handling changes, this becomes:
do {
struct defrag_segment segment;
ret = defrag_get_next_segment(defrag_fd, &segment);
} while (ret == 0);
if (ret == 0 || ret == -ENODATA)
exit_value = 0;
else
exit_value = 1;
Ok, so this is a linear iteration of all extents in the file that
filters extents for the specific "segment" that is going to be
processed. I still have no idea why fixed length segments are
important, but "linear extent scan for filtering" seems somewhat
expensive.
Indeed, if you used FIEMAP, you can pass a minimum
segment length to filter out all the small extents. Iterating that
extent list means all the ranges you need to defrag are in the holes
of the returned mapping information. This would be much faster
than an entire linear mapping to find all the regions with small
extents that need defrag. The second step could then be doing a
fine grained mapping of each region that we now know either contains
fragmented data or holes....
-Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
