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__write_sb_page() rounds up the io size to the optimal io size if it doesn't exceed the data offset, but it doesn't check the final size exceeds the bitmap length. For example: page count - 1 page size - 4K data offset - 1M optimal io size - 256K The final io size would be 256K (64 pages) but md_bitmap_storage_alloc() allocated 1 page, the IO would write 1 valid page and 63 pages that happens to be allocated afterwards. This leaks memory to the raid device superblock. This issue caused a data transfer failure in nvme-tcp. The network drivers checks the first page of an IO with sendpage_ok(), it returns true if the page isn't a slabpage and refcount >= 1. If the page !sendpage_ok() the network driver disables MSG_SPLICE_PAGES. As of now the network layer assumes all the pages of the IO are sendpage_ok() when MSG_SPLICE_PAGES is on. The bitmap pages aren't slab pages, the first page of the IO is sendpage_ok(), but the additional pages that happens to be allocated after the bitmap pages might be !sendpage_ok(). That cause skb_splice_from_iter() to stop the data transfer, in the case below it hangs 'mdadm --create'. The bug is reproducible, in order to reproduce we need nvme-over-tcp controllers with optimal IO size bigger than PAGE_SIZE. Creating a raid with bitmap over those devices reproduces the bug. In order to simulate large optimal IO size you can use dm-stripe with a single device. Script to reproduce the issue on top of brd devices using dm-stripe is attached below (will be added to blktest). I have added some logs to test the theory: ... md: created bitmap (1 pages) for device md127 __write_sb_page before md_super_write offset: 16, size: 262144. pfn: 0x53ee === __write_sb_page before md_super_write. logging pages === pfn: 0x53ee, slab: 0 <-- the only page that allocated for the bitmap pfn: 0x53ef, slab: 1 pfn: 0x53f0, slab: 0 pfn: 0x53f1, slab: 0 pfn: 0x53f2, slab: 0 pfn: 0x53f3, slab: 1 ... nvme_tcp: sendpage_ok - pfn: 0x53ee, len: 262144, offset: 0 skbuff: before sendpage_ok() - pfn: 0x53ee skbuff: before sendpage_ok() - pfn: 0x53ef WARNING at net/core/skbuff.c:6848 skb_splice_from_iter+0x142/0x450 skbuff: !sendpage_ok - pfn: 0x53ef. is_slab: 1, page_count: 1 ... Cc: stable@vger.kernel.org Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Ofir Gal <ofir.gal@volumez.com> Signed-off-by: Song Liu <song@kernel.org> Link: https://lore.kernel.org/r/20240607072748.3182199-1-ofir.gal@volumez.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.