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The following commit(s) were added to refs/heads/master by this push:
new 6ef3b1ba96 avformat/shared: remove MMAP fast path
6ef3b1ba96 is described below
commit 6ef3b1ba966233a0964bcc4aa1c2e20d014a5052
Author: Niklas Haas <[email protected]>
AuthorDate: Tue Sep 29 20:35:46 2026 +0200
Commit: Niklas Haas <[email protected]>
CommitDate: Mon Oct 12 14:34:53 2026 +0200
avformat/shared: remove MMAP fast path
The gain of this fast path is a bit of memory bandwidth for unaligned acesss
(for the read CRC check on a cache hit, and the writeback on cache miss).
The cost, apart from being more brittle (e.g. on filesystems that handle
mapped sparse files poorly), is that it prevents us from handling the
out-of-space (ENOSPC) case gracefully.
Using the pwrite() path unconditionally is safer and degrades this SIGBUS
into a simple AVERROR(ENOSPC), which is already handled gracefully by the
write_error fallback path.
Sponsored-by: nxtedition AB
Closes: https://github.com/nxtedition/FFmpeg/issues/81
Signed-off-by: Niklas Haas <[email protected]>
---
libavformat/shared.c | 84 +++++++---------------------------------------------
1 file changed, 11 insertions(+), 73 deletions(-)
diff --git a/libavformat/shared.c b/libavformat/shared.c
index 4f438d8e7c..aac76683ab 100644
--- a/libavformat/shared.c
+++ b/libavformat/shared.c
@@ -185,9 +185,7 @@ typedef struct SharedContext {
int64_t blocks_max; ///< maximum number of blocks to cache
/* cache file */
- uint8_t *cache_data; ///< optional mapping of the cache file
char *cache_path;
- off_t cache_size; ///< size of mapped memory region (for unmapping)
int fd;
/* space map */
@@ -206,7 +204,6 @@ static int shared_close(URLContext *h)
SharedContext *s = h->priv_data;
ffurl_close(s->inner);
- av_file_unmap_shared(s->cache_data, s->cache_size);
av_file_unmap_shared(s->spacemap, s->map_size);
if (s->fd != -1)
close(s->fd);
@@ -221,7 +218,6 @@ static int shared_close(URLContext *h)
return 0;
}
-static int cache_map(URLContext *h, int64_t filesize);
static int spacemap_init(URLContext *h, const uint8_t hash[HASH_SIZE]);
static int spacemap_grow(URLContext *h, int64_t block);
@@ -253,10 +249,6 @@ static int set_filesize(URLContext *h, int64_t new_size)
"%"PRId64", got: %"PRIu64"!\n", new_size,
(uint64_t) atomic_load(&s->spacemap->filesize));
return ret;
- } else if (ret) {
- /* Opportunistically map the file; this also sets the correct filesize.
- * Ignore errors as this is not critical to the cache logic. */
- cache_map(h, new_size);
}
return ret;
@@ -361,14 +353,6 @@ static int shared_open(URLContext *h, const char *arg, int
flags, AVDictionary *
ret = spacemap_grow(h, last_block);
if (ret < 0)
goto fail;
-
- /* If filesize is known, we can directly map the cache file */
- ret = cache_map(h, filesize);
- if (ret < 0) {
- av_log(h, AV_LOG_WARNING, "Failed to map cache file: %s. Falling "
- "back to normal read/write\n", av_err2str(ret));
- ret = 0;
- }
}
/* Temporary buffer needed for pread/pwrite() fallback */
@@ -388,31 +372,6 @@ fail:
return ret;
}
-static int cache_map(URLContext *h, int64_t filesize)
-{
- SharedContext *s = h->priv_data;
- if (s->cache_size >= filesize || filesize > SIZE_MAX)
- return 0;
-
- if (s->cache_data) {
- av_file_unmap_shared(s->cache_data, s->cache_size);
- s->cache_data = NULL;
- s->cache_size = 0;
- }
-
- /* The mapping extends the file to the file size; it can be shorter if
- * another process wrote the correct filesize to the header but crashed
- * right before actually successfully resizing the file. */
- void *map;
- int ret = av_file_map_shared(s->fd, filesize, &map);
- if (ret < 0)
- return ret;
-
- s->cache_data = map;
- s->cache_size = filesize;
- return 0;
-}
-
static int spacemap_remap(URLContext *h, size_t map_size)
{
SharedContext *s = h->priv_data;
@@ -573,12 +532,6 @@ static int spacemap_init(URLContext *h, const uint8_t
hash[HASH_SIZE])
static int read_cache(SharedContext *s, uint8_t *buf, size_t size, off_t
offset)
{
- if (s->cache_data) {
- av_assert1(offset + size <= s->cache_size);
- memcpy(buf, s->cache_data + offset, size);
- return 0;
- }
-
while (size) {
ssize_t ret = pread(s->fd, buf, size, offset);
if (ret <= 0)
@@ -593,12 +546,6 @@ static int read_cache(SharedContext *s, uint8_t *buf,
size_t size, off_t offset)
static int write_cache(SharedContext *s, const uint8_t *buf, size_t size,
off_t offset)
{
- if (s->cache_data) {
- av_assert1(offset + size <= s->cache_size);
- memcpy(s->cache_data + offset, buf, size);
- return 0;
- }
-
while (size) {
ssize_t ret = pwrite(s->fd, buf, size, offset);
if (ret <= 0)
@@ -666,23 +613,18 @@ retry:
/* We always need to read the entire block to verify integrity */
block_size = clamp_size(h, block_size, block_pos, filesize);
- if (s->cache_data) {
- av_assert1(block_pos + block_size <= s->cache_size);
- tmp = s->cache_data + block_pos;
- } else {
- tmp = s->tmp_buf;
- ret = read_cache(s, tmp, block_size, block_pos);
- if (ret < 0) {
- av_log(h, AV_LOG_ERROR, "Failed to read from cache file:
%s\n", av_err2str(ret));
- if (ret == AVERROR_EOF) { /* e.g. cache appears truncated? */
- if (s->retry_corrupt) {
- s->num_corrupt++;
- goto read_block;
- }
- ret = AVERROR(EIO); /* don't propagate EOF to caller */
+ tmp = s->tmp_buf;
+ ret = read_cache(s, tmp, block_size, block_pos);
+ if (ret < 0) {
+ av_log(h, AV_LOG_ERROR, "Failed to read from cache file: %s\n",
av_err2str(ret));
+ if (ret == AVERROR_EOF) { /* e.g. cache appears truncated? */
+ if (s->retry_corrupt) {
+ s->num_corrupt++;
+ goto read_block;
}
- return ret;
+ ret = AVERROR(EIO); /* don't propagate EOF to caller */
}
+ return ret;
}
uint32_t crc = get_block_crc(tmp, block_size);
@@ -838,11 +780,7 @@ read_block:
}
int write_back = 1;
- if (s->cache_data && acquired) {
- /* Read directly into memory mapped cache file */
- tmp = s->cache_data + block_pos;
- write_back = 0;
- } else if (size >= block_size && !offset) {
+ if (size >= block_size && !offset) {
/* Read directly into output buffer if aligned and large enough */
tmp = buf;
} else {
--
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