Re: [PATCH] btrfs: do more graceful error/warning for 32bit kernel

[Qu Wenruo]

On 2021/2/25 下午11:34, David Sterba wrote:

On Thu, Feb 25, 2021 at 07:44:19AM +0800, Qu Wenruo wrote:

On 2021/2/25 上午3:18, David Sterba wrote:

On Sat, Feb 20, 2021 at 10:06:33AM +0800, Qu Wenruo wrote:

Due to the pagecache limit of 32bit systems, btrfs can't access metadata
at or beyond 16T boundary correctly.

And unlike other fses, btrfs uses internally mapped u64 address space for
all of its metadata, this is more tricky than other fses.

Users can have a fs which doesn't have metadata beyond 16T boundary at
mount time, but later balance can cause btrfs to create metadata beyond
16T boundary.

As this is for the interhal logical offsets, it should be fixable by
reusing the range below 16T on 32bit systems. There's some logic relying
on the highest logical offset and block group flags so this needs to be
done with some care, but is possible in principle.

I doubt, as with the dropping price per-GB, user can still have extreme
case where all metadata goes beyond 16T in size.

But unlikely on a 32bit machine. And if yes we'll have the warnings in
place, as a stop gap.

The proper fix may be multiple metadata address spaces for 32bit
systems, but that would bring extra problems too.

Finally it doesn't really solve the problem that we don't have enough
test coverage for 32 bit at all.

That's true and it'll be worse as distributions drop 32bit builds. There
are stil non-intel arches that slowly get the 64bit CPUs but such
machines are not likely to have huge storage attached. Vendors of NAS
boxes patch their kernels anyway.

So for now I still believe we should just reject and do early warning.

I agree.

[...]

+#if BITS_PER_LONG == 32
+#define BTRFS_32BIT_EARLY_WARN_THRESHOLD       (10ULL * 1024 * SZ_1G)

Although the threshold should be calculated based on page size, not a
fixed value.

Would it make a difference? I think setting the early warning to 10T
sounds reasonable in all cases. IMHO you could keep it as is.

The problem is page size.

If we have 64K page size, the file size limit would be 256T, and then
10T threshold is definitely too low.

So in the submitted v2, the threshold is changed to 5/8 of the max file
size.

Thanks,
Qu

[...]

+#if BITS_PER_LONG == 32
+void __cold btrfs_warn_32bit_limit(struct btrfs_fs_info *fs_info)
+{
+       if (!test_and_set_bit(BTRFS_FS_32BIT_WARN, &fs_info->flags)) {
+               btrfs_warn(fs_info, "btrfs is reaching 32bit kernel limit.");
+               btrfs_warn(fs_info,
+"due to 32bit page cache limit, btrfs can't access metadata at or beyond 
16T.");

Also for the limit.

Thanks,
Qu

+               btrfs_warn(fs_info,
+                          "please consider upgrade to 64bit kernel/hardware.");
+       }
+}
+
+void __cold btrfs_err_32bit_limit(struct btrfs_fs_info *fs_info)
+{
+       if (!test_and_set_bit(BTRFS_FS_32BIT_ERROR, &fs_info->flags)) {
+               btrfs_err(fs_info, "btrfs reached 32bit kernel limit.");
+               btrfs_err(fs_info,
+"due to 32bit page cache limit, btrfs can't access metadata at or beyond 
16T.");
+               btrfs_err(fs_info,
+                          "please consider upgrade to 64bit kernel/hardware.");
+       }
+}
+#endif
+
   /*
    * We only mark the transaction aborted and then set the file system 
read-only.
    * This will prevent new transactions from starting or trying to join this
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index b8fab44394f5..5dc22daa684d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6787,6 +6787,46 @@ static u64 calc_stripe_length(u64 type, u64 chunk_len, 
int num_stripes)
        return div_u64(chunk_len, data_stripes);
   }

+#if BITS_PER_LONG == 32
+/*
+ * Due to page cache limit, btrfs can't access metadata at or beyond
+ * MAX_LFS_FILESIZE (16T) on 32bit systemts.
+ *
+ * This function do mount time check to reject the fs if it already has
+ * metadata chunk beyond that limit.
+ */
+static int check_32bit_meta_chunk(struct btrfs_fs_info *fs_info,
+                                 u64 logical, u64 length, u64 type)
+{
+       if (!(type & BTRFS_BLOCK_GROUP_METADATA))
+               return 0;
+
+       if (logical + length < MAX_LFS_FILESIZE)
+               return 0;
+
+       btrfs_err_32bit_limit(fs_info);
+       return -EOVERFLOW;
+}
+
+/*
+ * This is to give early warning for any metadata chunk reaching
+ * 10T boundary.
+ * Although we can still access the metadata, it's a timed bomb thus an early
+ * warning is definitely needed.
+ */
+static void warn_32bit_meta_chunk(struct btrfs_fs_info *fs_info,
+                                 u64 logical, u64 length, u64 type)
+{
+       if (!(type & BTRFS_BLOCK_GROUP_METADATA))
+               return;
+
+       if (logical + length < BTRFS_32BIT_EARLY_WARN_THRESHOLD)
+               return;
+
+       btrfs_warn_32bit_limit(fs_info);
+}
+#endif
+
   static int read_one_chunk(struct btrfs_key *key, struct extent_buffer *leaf,
                          struct btrfs_chunk *chunk)
   {
@@ -6797,6 +6837,7 @@ static int read_one_chunk(struct btrfs_key *key, struct 
extent_buffer *leaf,
        u64 logical;
        u64 length;
        u64 devid;
+       u64 type;
        u8 uuid[BTRFS_UUID_SIZE];
        int num_stripes;
        int ret;
@@ -6804,8 +6845,17 @@ static int read_one_chunk(struct btrfs_key *key, struct 
extent_buffer *leaf,

        logical = key->offset;
        length = btrfs_chunk_length(leaf, chunk);
+       type = btrfs_chunk_type(leaf, chunk);
        num_stripes = btrfs_chunk_num_stripes(leaf, chunk);

+#if BITS_PER_LONG == 32
+       ret = check_32bit_meta_chunk(fs_info, logical, length, type);
+       if (ret < 0)
+               return ret;
+       warn_32bit_meta_chunk(fs_info, logical, length, type);
+#endif
+
+
        /*
         * Only need to verify chunk item if we're reading from sys chunk array,
         * as chunk item in tree block is already verified by tree-checker.
@@ -6849,10 +6899,10 @@ static int read_one_chunk(struct btrfs_key *key, struct 
extent_buffer *leaf,
        map->io_width = btrfs_chunk_io_width(leaf, chunk);
        map->io_align = btrfs_chunk_io_align(leaf, chunk);
        map->stripe_len = btrfs_chunk_stripe_len(leaf, chunk);
-       map->type = btrfs_chunk_type(leaf, chunk);
+       map->type = type;
        map->sub_stripes = btrfs_chunk_sub_stripes(leaf, chunk);
        map->verified_stripes = 0;
-       em->orig_block_len = calc_stripe_length(map->type, em->len,
+       em->orig_block_len = calc_stripe_length(type, em->len,
                                                map->num_stripes);
        for (i = 0; i < num_stripes; i++) {
                map->stripes[i].physical =
--
2.30.0