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When tcmu_vma_fault() gets a page successfully, before the current context completes page fault procedure, find_free_blocks() may run and call unmap_mapping_range() to unmap the page. Assume that when find_free_blocks() initially completes and the previous page fault procedure starts to run again and completes, then one truncated page has been mapped to userspace. But note that tcmu_vma_fault() has gotten a refcount for the page so any other subsystem won't be able to use the page unless the userspace address is unmapped later. If another command subsequently runs and needs to extend dbi_thresh it may reuse the corresponding slot for the previous page in data_bitmap. Then though we'll allocate new page for this slot in data_area, no page fault will happen because we have a valid map and the real request's data will be lost. Filesystem implementations will also run into this issue but they usually lock the page when vm_operations_struct->fault gets a page and unlock the page after finish_fault() completes. For truncate filesystems lock pages in truncate_inode_pages() to protect against racing wrt. page faults. To fix this possible data corruption scenario we can apply a method similar to the filesystems. For pages that are to be freed, tcmu_blocks_release() locks and unlocks. Make tcmu_vma_fault() also lock found page under cmdr_lock. At the same time, since tcmu_vma_fault() gets an extra page refcount, tcmu_blocks_release() won't free pages if pages are in page fault procedure, which means it is safe to call tcmu_blocks_release() before unmap_mapping_range(). With these changes tcmu_blocks_release() will wait for all page faults to be completed before calling unmap_mapping_range(). And later, if unmap_mapping_range() is called, it will ensure stale mappings are removed. Link: https://lore.kernel.org/r/20220421023735.9018-1-xiaoguang.wang@linux.alibaba.com Reviewed-by: Bodo Stroesser <bostroesser@gmail.com> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> |
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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 Restructured Text 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.