forked from luck/tmp_suning_uos_patched
6b4c9f4469
Currently we only drop the mmap_sem if there is contention on the page lock. The idea is that we issue readahead and then go to lock the page while it is under IO and we want to not hold the mmap_sem during the IO. The problem with this is the assumption that the readahead does anything. In the case that the box is under extreme memory or IO pressure we may end up not reading anything at all for readahead, which means we will end up reading in the page under the mmap_sem. Even if the readahead does something, it could get throttled because of io pressure on the system and the process is in a lower priority cgroup. Holding the mmap_sem while doing IO is problematic because it can cause system-wide priority inversions. Consider some large company that does a lot of web traffic. This large company has load balancing logic in it's core web server, cause some engineer thought this was a brilliant plan. This load balancing logic gets statistics from /proc about the system, which trip over processes mmap_sem for various reasons. Now the web server application is in a protected cgroup, but these other processes may not be, and if they are being throttled while their mmap_sem is held we'll stall, and cause this nice death spiral. Instead rework filemap fault path to drop the mmap sem at any point that we may do IO or block for an extended period of time. This includes while issuing readahead, locking the page, or needing to call ->readpage because readahead did not occur. Then once we have a fully uptodate page we can return with VM_FAULT_RETRY and come back again to find our nicely in-cache page that was gotten outside of the mmap_sem. This patch also adds a new helper for locking the page with the mmap_sem dropped. This doesn't make sense currently as generally speaking if the page is already locked it'll have been read in (unless there was an error) before it was unlocked. However a forthcoming patchset will change this with the ability to abort read-ahead bio's if necessary, making it more likely that we could contend for a page lock and still have a not uptodate page. This allows us to deal with this case by grabbing the lock and issuing the IO without the mmap_sem held, and then returning VM_FAULT_RETRY to come back around. [josef@toxicpanda.com: v6] Link: http://lkml.kernel.org/r/20181212152757.10017-1-josef@toxicpanda.com [kirill@shutemov.name: fix race in filemap_fault()] Link: http://lkml.kernel.org/r/20181228235106.okk3oastsnpxusxs@kshutemo-mobl1 [akpm@linux-foundation.org: coding style fixes] Link: http://lkml.kernel.org/r/20181211173801.29535-4-josef@toxicpanda.com Signed-off-by: Josef Bacik <josef@toxicpanda.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Jan Kara <jack@suse.cz> Tested-by: syzbot+b437b5a429d680cf2217@syzkaller.appspotmail.com Cc: Dave Chinner <david@fromorbit.com> Cc: Rik van Riel <riel@redhat.com> Cc: Tejun Heo <tj@kernel.org> Cc: "Kirill A. Shutemov" <kirill@shutemov.name> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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.