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When re-using the CMA area for kdump there is a risk of pending DMA into pinned user pages in the CMA area. Pages residing in CMA areas can usually not get long-term pinned and are instead migrated away from the CMA area, so long-term pinning is typically not a concern. (BUGs in the kernel might still lead to long-term pinning of such pages if everything goes wrong.) Pages pinned without FOLL_LONGTERM remain in the CMA and may possibly be the source or destination of a pending DMA transfer. Although there is no clear specification how long a page may be pinned without FOLL_LONGTERM, pinning without the flag shows an intent of the caller to only use the memory for short-lived DMA transfers, not a transfer initiated by a device asynchronously at a random time in the future. Add a delay of CMA_DMA_TIMEOUT_SEC seconds before starting the kdump kernel, giving such short-lived DMA transfers time to finish before the CMA memory is re-used by the kdump kernel. Set CMA_DMA_TIMEOUT_SEC to 10 seconds - chosen arbitrarily as both a huge margin for a DMA transfer, yet not increasing the kdump time too significantly. Link: https://lkml.kernel.org/r/aEqpgDIBndZ5LXSo@dwarf.suse.cz Signed-off-by: Jiri Bohac <jbohac@suse.cz> Acked-by: David Hildenbrand <david@redhat.com> Cc: Baoquan He <bhe@redhat.com> Cc: Dave Young <dyoung@redhat.com> Cc: Donald Dutile <ddutile@redhat.com> Cc: Michal Hocko <mhocko@suse.cz> Cc: Philipp Rudo <prudo@redhat.com> Cc: Pingfan Liu <piliu@redhat.com> Cc: Tao Liu <ltao@redhat.com> Cc: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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.