Re: [PATCH] btrfs: Disable BTRFS on platforms having 256K pages

On Jun 11, 2021, at 9:21 AM, David Sterba [off-list ref] wrote:

On Fri, Jun 11, 2021 at 12:58:58PM +0000, Chris Mason wrote:

quoted

quoted

On Jun 10, 2021, at 12:20 PM, David Sterba [off-list ref] wrote:
On Thu, Jun 10, 2021 at 04:50:09PM +0200, Christophe Leroy wrote:

quoted

Le 10/06/2021 à 15:54, Chris Mason a écrit :

quoted

quoted

On Jun 10, 2021, at 1:23 AM, Christophe Leroy [off-list ref] wrote:

And there's no such thing like "just bump BTRFS_MAX_COMPRESSED to 256K".
The constant is part of on-disk format for lzo and otherwise changing it
would impact performance so this would need proper evaluation.

Sorry, how is it baked into LZO?  It definitely will have performance implications, I agree there.

lzo_decompress_bio:

309         /*
310          * Compressed data header check.
311          *
312          * The real compressed size can't exceed the maximum extent length, and
313          * all pages should be used (whole unused page with just the segment
314          * header is not possible).  If this happens it means the compressed
315          * extent is corrupted.
316          */
317         if (tot_len > min_t(size_t, BTRFS_MAX_COMPRESSED, srclen) ||
318             tot_len < srclen - PAGE_SIZE) {
319                 ret = -EUCLEAN;
320                 goto done;
321         }

Ah I see, so going back to an old LZO kernel will get upset.  Ok, fair enough.  So if we want to bump this for other reasons, we’ll need to make an LZO max size to maintain compatibility.

-chris