Fork of alistair23 Linux kernel for reMarkable from https://github.com/alistair23/linux
ea2c3f6f55
When using mremap() syscall in addition to MREMAP_FIXED flag, mremap() calls mremap_to() which does the following: 1) unmaps the destination region where we are going to move the map 2) If the new region is going to be smaller, we unmap the last part of the old region Then, we will eventually call move_vma() to do the actual move. move_vma() checks whether we are at least 4 maps below max_map_count before going further, otherwise it bails out with -ENOMEM. The problem is that we might have already unmapped the vma's in steps 1) and 2), so it is not possible for userspace to figure out the state of the vmas after it gets -ENOMEM, and it gets tricky for userspace to clean up properly on error path. While it is true that we can return -ENOMEM for more reasons (e.g: see may_expand_vm() or move_page_tables()), I think that we can avoid this scenario if we check early in mremap_to() if the operation has high chances to succeed map-wise. Should that not be the case, we can bail out before we even try to unmap anything, so we make sure the vma's are left untouched in case we are likely to be short of maps. The thumb-rule now is to rely on the worst-scenario case we can have. That is when both vma's (old region and new region) are going to be split in 3, so we get two more maps to the ones we already hold (one per each). If current map count + 2 maps still leads us to 4 maps below the threshold, we are going to pass the check in move_vma(). Of course, this is not free, as it might generate false positives when it is true that we are tight map-wise, but the unmap operation can release several vma's leading us to a good state. Another approach was also investigated [1], but it may be too much hassle for what it brings. [1] https://lore.kernel.org/lkml/20190219155320.tkfkwvqk53tfdojt@d104.suse.de/ Link: http://lkml.kernel.org/r/20190226091314.18446-1-osalvador@suse.de Signed-off-by: Oscar Salvador <osalvador@suse.de> Acked-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Hugh Dickins <hughd@google.com> Cc: Joel Fernandes (Google) <joel@joelfernandes.org> Cc: Yang Shi <yang.shi@linux.alibaba.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Joel Fernandes <joel@joelfernandes.org> Cc: Cyril Hrubis <chrubis@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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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.