ARC: Drop different size headers for crypto #15347
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I think this kind of simplification is easily worth the 40 bytes per header (or more). Looking through the patch everything looks reasonable but I haven't tested it myself. |
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I've rebased this on top of #15350. No other changes intended. |
In most cases we do not care about exact number of buffers linked to the header, we just need to know if it is zero, non-zero or one. That can easily be checked just looking on b_buf pointer or in some cases derefencing it. b_ebufcnt is read only once, and in that case we already traverse the list as part of arc_buf_remove(), so second traverse should not be expensive. This reduces L1 ARC header size by 8 bytes and full crypto header by 16 bytes, down to 176 and 232 bytes on FreeBSD respectively. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
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To reduce memory usage ZFS crypto allocated bigger by 56 bytes ARC headers only when specific block was encrypted on disk. It was a nice optimization, except in some cases the code reallocated them on fly, that invalidated header pointers from the buffers. Since the buffers use different locking, it created number of races, that were originally covered (at least partially) by b_evict_lock, used also to protection evictions. But it has gone as part of openzfs#14340. As result, as was found in openzfs#15293, arc_hdr_realloc_crypt() ended up unprotected and causing use-after-free. Instead of introducing some even more elaborate locking, this patch just drops the difference between normal and protected headers. It cost us additional 56 bytes per header, but with couple patches saving 24 bytes, the net growth is only 32 bytes with total header size of 232 bytes on FreeBSD, that IMHO is acceptable price for simplicity. Additional locking would also end up consuming space, time or both. Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc.
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Rebased on top of updated #15350. Now the full cost is 56 bytes, and net is only 32. |
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To reduce memory usage ZFS crypto allocated bigger by 56 bytes ARC headers only when specific block was encrypted on disk. It was a nice optimization, except in some cases the code reallocated them on fly, that invalidated header pointers from the buffers. Since the buffers use different locking, it created number of races, that were originally covered (at least partially) by b_evict_lock, used also to protection evictions. But it has gone as part of openzfs#14340. As result, as was found in openzfs#15293, arc_hdr_realloc_crypt() ended up unprotected and causing use-after-free. Instead of introducing some even more elaborate locking, this patch just drops the difference between normal and protected headers. It cost us additional 56 bytes per header, but with couple patches saving 24 bytes, the net growth is only 32 bytes with total header size of 232 bytes on FreeBSD, that IMHO is acceptable price for simplicity. Additional locking would also end up consuming space, time or both. Reviewe-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15293 Closes openzfs#15347
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To reduce memory usage ZFS crypto allocated bigger by 56 bytes ARC headers only when specific block was encrypted on disk. It was a nice optimization, except in some cases the code reallocated them on fly, that invalidated header pointers from the buffers. Since the buffers use different locking, it created number of races, that were originally covered (at least partially) by b_evict_lock, used also to protection evictions. But it has gone as part of #14340. As result, as was found in #15293, arc_hdr_realloc_crypt() ended up unprotected and causing use-after-free. Instead of introducing some even more elaborate locking, this patch just drops the difference between normal and protected headers. It cost us additional 56 bytes per header, but with couple patches saving 24 bytes, the net growth is only 32 bytes with total header size of 232 bytes on FreeBSD, that IMHO is acceptable price for simplicity. Additional locking would also end up consuming space, time or both. Reviewe-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes #15293 Closes #15347
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To reduce memory usage ZFS crypto allocated bigger by 56 bytes ARC headers only when specific block was encrypted on disk. It was a nice optimization, except in some cases the code reallocated them on fly, that invalidated header pointers from the buffers. Since the buffers use different locking, it created number of races, that were originally covered (at least partially) by b_evict_lock, used also to protection evictions. But it has gone as part of openzfs#14340. As result, as was found in openzfs#15293, arc_hdr_realloc_crypt() ended up unprotected and causing use-after-free. Instead of introducing some even more elaborate locking, this patch just drops the difference between normal and protected headers. It cost us additional 56 bytes per header, but with couple patches saving 24 bytes, the net growth is only 32 bytes with total header size of 232 bytes on FreeBSD, that IMHO is acceptable price for simplicity. Additional locking would also end up consuming space, time or both. Reviewe-by: Brian Behlendorf <[email protected]> Signed-off-by: Alexander Motin <[email protected]> Sponsored by: iXsystems, Inc. Closes openzfs#15293 Closes openzfs#15347
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To reduce memory usage ZFS crypto allocated bigger by 56 bytes ARC headers only when specific block was encrypted on disk. It was a nice optimization, except in some cases the code reallocated them on fly, that invalidated header pointers from the buffers. Since the buffers use different locking, it created number of races, that were originally covered (at least partially) by b_evict_lock, used also to protect evictions. But it has gone as part of #14340. As result, as was found in #15293, arc_hdr_realloc_crypt() ended up unprotected and causing use-after-free.
Instead of introducing some even more elaborate locking, this patch just drops the difference between normal and protected headers. It cost us additional 56 bytes per header, but with couple patches saving 24 bytes, the net growth is only 32 bytes with total header size of 232 bytes on FreeBSD, that IMHO is acceptable price for simplicity. Additional locking would also end up consuming space, time or both.
This PR conflicts with #15350, so for now includes them as separate commits.
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