Re: [Qemu-devel] [PATCH v3 1/5] 9p: mitigates most QID path collisions

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Apr 23, 2019 at 01:30:03PM +0200, Christian Schoenebeck via Qemu-devel wrote:
> This first patch here is an updated version of Antonios Motakis'
> original 4-patch set (using fixed length 16 bit prefixes), merged to one
> patch:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg02283.html
> 
> * Updated to latest git master, specifically to new qht interface.
> 
> * Merged the original 4 patches to this single patch.

Why did you merge them all into one ?  The split patches were "best
practice" IMHO. The original patch authorship & S-o-B lines could
be preserved if you kept them split as before too.

> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  fsdev/9p-marshal.h |   4 +-
>  hw/9pfs/9p.c       | 200 ++++++++++++++++++++++++++++++++++++++++++++++++-----
>  hw/9pfs/9p.h       |  21 ++++++
>  3 files changed, 204 insertions(+), 21 deletions(-)
> 
> diff --git a/fsdev/9p-marshal.h b/fsdev/9p-marshal.h
> index c8823d878f..d1ad3645c4 100644
> --- a/fsdev/9p-marshal.h
> +++ b/fsdev/9p-marshal.h
> @@ -10,8 +10,8 @@ typedef struct V9fsString
>  typedef struct V9fsQID
>  {
>      int8_t type;
> -    int32_t version;
> -    int64_t path;
> +    uint32_t version;
> +    uint64_t path;
>  } V9fsQID;
>  
>  typedef struct V9fsStat
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 55821343e5..b9bbdcbaee 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -25,6 +25,7 @@
>  #include "trace.h"
>  #include "migration/blocker.h"
>  #include "sysemu/qtest.h"
> +#include "qemu/xxhash.h"
>  
>  int open_fd_hw;
>  int total_open_fd;
> @@ -571,14 +572,135 @@ static void coroutine_fn virtfs_reset(V9fsPDU *pdu)
>                                  P9_STAT_MODE_NAMED_PIPE |   \
>                                  P9_STAT_MODE_SOCKET)
>  
> -/* This is the algorithm from ufs in spfs */
> -static void stat_to_qid(const struct stat *stbuf, V9fsQID *qidp)
> +
> +/* creative abuse of qemu_xxhash7, which is based on xxhash */
> +static uint32_t qpp_hash(QppEntry e)
>  {
> -    size_t size;
> +    return qemu_xxhash7(e.ino_prefix, e.dev, 0, 0, 0);
> +}
> +
> +static uint32_t qpf_hash(QpfEntry e)
> +{
> +    return qemu_xxhash7(e.ino, e.dev, 0, 0, 0);
> +}
> +
> +static bool qpp_cmp_func(const void *obj, const void *userp)
> +{
> +    const QppEntry *e1 = obj, *e2 = userp;
> +    return (e1->dev == e2->dev) && (e1->ino_prefix == e2->ino_prefix);
> +}
> +
> +static bool qpf_cmp_func(const void *obj, const void *userp)
> +{
> +    const QpfEntry *e1 = obj, *e2 = userp;
> +    return (e1->dev == e2->dev) && (e1->ino == e2->ino);
> +}
> +
> +static void qp_table_remove(void *p, uint32_t h, void *up)
> +{
> +    g_free(p);
> +}
> +
> +static void qp_table_destroy(struct qht *ht)
> +{
> +    qht_iter(ht, qp_table_remove, NULL);
> +    qht_destroy(ht);
> +}
> +
> +static int qid_path_fullmap(V9fsPDU *pdu, const struct stat *stbuf,
> +                            uint64_t *path)
> +{
> +    QpfEntry lookup = {
> +        .dev = stbuf->st_dev,
> +        .ino = stbuf->st_ino
> +    }, *val;
> +    uint32_t hash = qpf_hash(lookup);
> +
> +    /* most users won't need the fullmap, so init the table lazily */
> +    if (!pdu->s->qpf_table.map) {
> +        qht_init(&pdu->s->qpf_table, qpf_cmp_func, 1 << 16, QHT_MODE_AUTO_RESIZE);
> +    }
> +
> +    val = qht_lookup(&pdu->s->qpf_table, &lookup, hash);
> +
> +    if (!val) {
> +        if (pdu->s->qp_fullpath_next == 0) {
> +            /* no more files can be mapped :'( */
> +            return -ENFILE;
> +        }
> +
> +        val = g_malloc0(sizeof(QppEntry));
> +        if (!val) {
> +            return -ENOMEM;
> +        }
> +        *val = lookup;
> +
> +        /* new unique inode and device combo */
> +        val->path = pdu->s->qp_fullpath_next++;
> +        pdu->s->qp_fullpath_next &= QPATH_INO_MASK;
> +        qht_insert(&pdu->s->qpf_table, val, hash, NULL);
> +    }
> +
> +    *path = val->path;
> +    return 0;
> +}
> +
> +/* stat_to_qid needs to map inode number (64 bits) and device id (32 bits)
> + * to a unique QID path (64 bits). To avoid having to map and keep track
> + * of up to 2^64 objects, we map only the 16 highest bits of the inode plus
> + * the device id to the 16 highest bits of the QID path. The 48 lowest bits
> + * of the QID path equal to the lowest bits of the inode number.
> + *
> + * This takes advantage of the fact that inode number are usually not
> + * random but allocated sequentially, so we have fewer items to keep
> + * track of.
> + */
> +static int qid_path_prefixmap(V9fsPDU *pdu, const struct stat *stbuf,
> +                                uint64_t *path)
> +{
> +    QppEntry lookup = {
> +        .dev = stbuf->st_dev,
> +        .ino_prefix = (uint16_t) (stbuf->st_ino >> 48)
> +    }, *val;
> +    uint32_t hash = qpp_hash(lookup);
> +
> +    val = qht_lookup(&pdu->s->qpp_table, &lookup, hash);
> +
> +    if (!val) {
> +        if (pdu->s->qp_prefix_next == 0) {
> +            /* we ran out of prefixes */
> +            return -ENFILE;
> +        }
> +
> +        val = g_malloc0(sizeof(QppEntry));
> +        if (!val) {
> +            return -ENOMEM;
> +        }
> +        *val = lookup;
> +
> +        /* new unique inode prefix and device combo */
> +        val->qp_prefix = pdu->s->qp_prefix_next++;
> +        qht_insert(&pdu->s->qpp_table, val, hash, NULL);
> +    }
> +
> +    *path = ((uint64_t)val->qp_prefix << 48) | (stbuf->st_ino & QPATH_INO_MASK);
> +    return 0;
> +}
> +
> +static int stat_to_qid(V9fsPDU *pdu, const struct stat *stbuf, V9fsQID *qidp)
> +{
> +    int err;
> +
> +    /* map inode+device to qid path (fast path) */
> +    err = qid_path_prefixmap(pdu, stbuf, &qidp->path);
> +    if (err == -ENFILE) {
> +        /* fast path didn't work, fal back to full map */
> +        err = qid_path_fullmap(pdu, stbuf, &qidp->path);
> +    }
> +    if (err) {
> +        return err;
> +    }
>  
> -    memset(&qidp->path, 0, sizeof(qidp->path));
> -    size = MIN(sizeof(stbuf->st_ino), sizeof(qidp->path));
> -    memcpy(&qidp->path, &stbuf->st_ino, size);
>      qidp->version = stbuf->st_mtime ^ (stbuf->st_size << 8);
>      qidp->type = 0;
>      if (S_ISDIR(stbuf->st_mode)) {
> @@ -587,6 +709,8 @@ static void stat_to_qid(const struct stat *stbuf, V9fsQID *qidp)
>      if (S_ISLNK(stbuf->st_mode)) {
>          qidp->type |= P9_QID_TYPE_SYMLINK;
>      }
> +
> +    return 0;
>  }
>  
>  static int coroutine_fn fid_to_qid(V9fsPDU *pdu, V9fsFidState *fidp,
> @@ -599,7 +723,10 @@ static int coroutine_fn fid_to_qid(V9fsPDU *pdu, V9fsFidState *fidp,
>      if (err < 0) {
>          return err;
>      }
> -    stat_to_qid(&stbuf, qidp);
> +    err = stat_to_qid(pdu, &stbuf, qidp);
> +    if (err < 0) {
> +        return err;
> +    }
>      return 0;
>  }
>  
> @@ -830,7 +957,10 @@ static int coroutine_fn stat_to_v9stat(V9fsPDU *pdu, V9fsPath *path,
>  
>      memset(v9stat, 0, sizeof(*v9stat));
>  
> -    stat_to_qid(stbuf, &v9stat->qid);
> +    err = stat_to_qid(pdu, stbuf, &v9stat->qid);
> +    if (err < 0) {
> +        return err;
> +    }
>      v9stat->mode = stat_to_v9mode(stbuf);
>      v9stat->atime = stbuf->st_atime;
>      v9stat->mtime = stbuf->st_mtime;
> @@ -891,7 +1021,7 @@ static int coroutine_fn stat_to_v9stat(V9fsPDU *pdu, V9fsPath *path,
>  #define P9_STATS_ALL           0x00003fffULL /* Mask for All fields above */
>  
>  
> -static void stat_to_v9stat_dotl(V9fsState *s, const struct stat *stbuf,
> +static int stat_to_v9stat_dotl(V9fsPDU *pdu, const struct stat *stbuf,
>                                  V9fsStatDotl *v9lstat)
>  {
>      memset(v9lstat, 0, sizeof(*v9lstat));
> @@ -913,7 +1043,7 @@ static void stat_to_v9stat_dotl(V9fsState *s, const struct stat *stbuf,
>      /* Currently we only support BASIC fields in stat */
>      v9lstat->st_result_mask = P9_STATS_BASIC;
>  
> -    stat_to_qid(stbuf, &v9lstat->qid);
> +    return stat_to_qid(pdu, stbuf, &v9lstat->qid);
>  }
>  
>  static void print_sg(struct iovec *sg, int cnt)
> @@ -1115,7 +1245,6 @@ static void coroutine_fn v9fs_getattr(void *opaque)
>      uint64_t request_mask;
>      V9fsStatDotl v9stat_dotl;
>      V9fsPDU *pdu = opaque;
> -    V9fsState *s = pdu->s;
>  
>      retval = pdu_unmarshal(pdu, offset, "dq", &fid, &request_mask);
>      if (retval < 0) {
> @@ -1136,7 +1265,10 @@ static void coroutine_fn v9fs_getattr(void *opaque)
>      if (retval < 0) {
>          goto out;
>      }
> -    stat_to_v9stat_dotl(s, &stbuf, &v9stat_dotl);
> +    retval = stat_to_v9stat_dotl(pdu, &stbuf, &v9stat_dotl);
> +    if (retval < 0) {
> +        goto out;
> +    }
>  
>      /*  fill st_gen if requested and supported by underlying fs */
>      if (request_mask & P9_STATS_GEN) {
> @@ -1381,7 +1513,10 @@ static void coroutine_fn v9fs_walk(void *opaque)
>              if (err < 0) {
>                  goto out;
>              }
> -            stat_to_qid(&stbuf, &qid);
> +            err = stat_to_qid(pdu, &stbuf, &qid);
> +            if (err < 0) {
> +                goto out;
> +            }
>              v9fs_path_copy(&dpath, &path);
>          }
>          memcpy(&qids[name_idx], &qid, sizeof(qid));
> @@ -1483,7 +1618,10 @@ static void coroutine_fn v9fs_open(void *opaque)
>      if (err < 0) {
>          goto out;
>      }
> -    stat_to_qid(&stbuf, &qid);
> +    err = stat_to_qid(pdu, &stbuf, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
>      if (S_ISDIR(stbuf.st_mode)) {
>          err = v9fs_co_opendir(pdu, fidp);
>          if (err < 0) {
> @@ -1593,7 +1731,10 @@ static void coroutine_fn v9fs_lcreate(void *opaque)
>          fidp->flags |= FID_NON_RECLAIMABLE;
>      }
>      iounit =  get_iounit(pdu, &fidp->path);
> -    stat_to_qid(&stbuf, &qid);
> +    err = stat_to_qid(pdu, &stbuf, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
>      err = pdu_marshal(pdu, offset, "Qd", &qid, iounit);
>      if (err < 0) {
>          goto out;
> @@ -2327,7 +2468,10 @@ static void coroutine_fn v9fs_create(void *opaque)
>          }
>      }
>      iounit = get_iounit(pdu, &fidp->path);
> -    stat_to_qid(&stbuf, &qid);
> +    err = stat_to_qid(pdu, &stbuf, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
>      err = pdu_marshal(pdu, offset, "Qd", &qid, iounit);
>      if (err < 0) {
>          goto out;
> @@ -2384,7 +2528,10 @@ static void coroutine_fn v9fs_symlink(void *opaque)
>      if (err < 0) {
>          goto out;
>      }
> -    stat_to_qid(&stbuf, &qid);
> +    err = stat_to_qid(pdu, &stbuf, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
>      err =  pdu_marshal(pdu, offset, "Q", &qid);
>      if (err < 0) {
>          goto out;
> @@ -3064,7 +3211,10 @@ static void coroutine_fn v9fs_mknod(void *opaque)
>      if (err < 0) {
>          goto out;
>      }
> -    stat_to_qid(&stbuf, &qid);
> +    err = stat_to_qid(pdu, &stbuf, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
>      err = pdu_marshal(pdu, offset, "Q", &qid);
>      if (err < 0) {
>          goto out;
> @@ -3222,7 +3372,10 @@ static void coroutine_fn v9fs_mkdir(void *opaque)
>      if (err < 0) {
>          goto out;
>      }
> -    stat_to_qid(&stbuf, &qid);
> +    err = stat_to_qid(pdu, &stbuf, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
>      err = pdu_marshal(pdu, offset, "Q", &qid);
>      if (err < 0) {
>          goto out;
> @@ -3633,6 +3786,11 @@ int v9fs_device_realize_common(V9fsState *s, const V9fsTransport *t,
>          goto out;
>      }
>  
> +    /* QID path hash table. 1 entry ought to be enough for anybody ;) */
> +    qht_init(&s->qpp_table, qpp_cmp_func, 1, QHT_MODE_AUTO_RESIZE);
> +    s->qp_prefix_next = 1; /* reserve 0 to detect overflow */
> +    s->qp_fullpath_next = 1;
> +
>      s->ctx.fst = &fse->fst;
>      fsdev_throttle_init(s->ctx.fst);
>  
> @@ -3646,6 +3804,8 @@ out:
>          }
>          g_free(s->tag);
>          g_free(s->ctx.fs_root);
> +        qp_table_destroy(&s->qpp_table);
> +        qp_table_destroy(&s->qpf_table);
>          v9fs_path_free(&path);
>      }
>      return rc;
> @@ -3658,6 +3818,8 @@ void v9fs_device_unrealize_common(V9fsState *s, Error **errp)
>      }
>      fsdev_throttle_cleanup(s->ctx.fst);
>      g_free(s->tag);
> +    qp_table_destroy(&s->qpp_table);
> +    qp_table_destroy(&s->qpf_table);
>      g_free(s->ctx.fs_root);
>  }
>  
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index 8883761b2c..44112ea97f 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -8,6 +8,7 @@
>  #include "fsdev/9p-iov-marshal.h"
>  #include "qemu/thread.h"
>  #include "qemu/coroutine.h"
> +#include "qemu/qht.h"
>  
>  enum {
>      P9_TLERROR = 6,
> @@ -235,6 +236,22 @@ struct V9fsFidState
>      V9fsFidState *rclm_lst;
>  };
>  
> +#define QPATH_INO_MASK        (((unsigned long)1 << 48) - 1)
> +
> +/* QID path prefix entry, see stat_to_qid */
> +typedef struct {
> +    dev_t dev;
> +    uint16_t ino_prefix;
> +    uint16_t qp_prefix;
> +} QppEntry;
> +
> +/* QID path full entry, as above */
> +typedef struct {
> +    dev_t dev;
> +    ino_t ino;
> +    uint64_t path;
> +} QpfEntry;
> +
>  struct V9fsState
>  {
>      QLIST_HEAD(, V9fsPDU) free_list;
> @@ -256,6 +273,10 @@ struct V9fsState
>      Error *migration_blocker;
>      V9fsConf fsconf;
>      V9fsQID root_qid;
> +    struct qht qpp_table;
> +    struct qht qpf_table;
> +    uint16_t qp_prefix_next;
> +    uint64_t qp_fullpath_next;
>  };
>  
>  /* 9p2000.L open flags */
> -- 
> 2.11.0
> 
> 

Regards,
Daniel
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