Commit 7a14239a authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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mm Documentation: undoc non-linear vmas



While updating some mm Documentation, I came across a few straggling
references to the non-linear vmas which were happily removed in v4.0.
Delete them.
Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 706874e9
......@@ -485,7 +485,6 @@ manner. The codes are the following:
ac - area is accountable
nr - swap space is not reserved for the area
ht - area uses huge tlb pages
nl - non-linear mapping
ar - architecture specific flag
dd - do not include area into core dump
sd - soft-dirty flag
......
......@@ -99,12 +99,10 @@ Steps:
4. The new page is prepped with some settings from the old page so that
accesses to the new page will discover a page with the correct settings.
5. All the page table references to the page are converted
to migration entries or dropped (nonlinear vmas).
This decrease the mapcount of a page. If the resulting
mapcount is not zero then we do not migrate the page.
All user space processes that attempt to access the page
will now wait on the page lock.
5. All the page table references to the page are converted to migration
entries. This decreases the mapcount of a page. If the resulting
mapcount is not zero then we do not migrate the page. All user space
processes that attempt to access the page will now wait on the page lock.
6. The radix tree lock is taken. This will cause all processes trying
to access the page via the mapping to block on the radix tree spinlock.
......
......@@ -552,63 +552,17 @@ different reverse map mechanisms.
is really unevictable or not. In this case, try_to_unmap_anon() will
return SWAP_AGAIN.
(*) try_to_unmap_file() - linear mappings
(*) try_to_unmap_file()
Unmapping of a mapped file page works the same as for anonymous mappings,
except that the scan visits all VMAs that map the page's index/page offset
in the page's mapping's reverse map priority search tree. It also visits
each VMA in the page's mapping's non-linear list, if the list is
non-empty.
in the page's mapping's reverse map interval search tree.
As for anonymous pages, on encountering a VM_LOCKED VMA for a mapped file
page, try_to_unmap_file() will attempt to acquire the associated
mm_struct's mmap semaphore to mlock the page, returning SWAP_MLOCK if this
is successful, and SWAP_AGAIN, if not.
(*) try_to_unmap_file() - non-linear mappings
If a page's mapping contains a non-empty non-linear mapping VMA list, then
try_to_un{map|lock}() must also visit each VMA in that list to determine
whether the page is mapped in a VM_LOCKED VMA. Again, the scan must visit
all VMAs in the non-linear list to ensure that the pages is not/should not
be mlocked.
If a VM_LOCKED VMA is found in the list, the scan could terminate.
However, there is no easy way to determine whether the page is actually
mapped in a given VMA - either for unmapping or testing whether the
VM_LOCKED VMA actually pins the page.
try_to_unmap_file() handles non-linear mappings by scanning a certain
number of pages - a "cluster" - in each non-linear VMA associated with the
page's mapping, for each file mapped page that vmscan tries to unmap. If
this happens to unmap the page we're trying to unmap, try_to_unmap() will
notice this on return (page_mapcount(page) will be 0) and return
SWAP_SUCCESS. Otherwise, it will return SWAP_AGAIN, causing vmscan to
recirculate this page. We take advantage of the cluster scan in
try_to_unmap_cluster() as follows:
For each non-linear VMA, try_to_unmap_cluster() attempts to acquire the
mmap semaphore of the associated mm_struct for read without blocking.
If this attempt is successful and the VMA is VM_LOCKED,
try_to_unmap_cluster() will retain the mmap semaphore for the scan;
otherwise it drops it here.
Then, for each page in the cluster, if we're holding the mmap semaphore
for a locked VMA, try_to_unmap_cluster() calls mlock_vma_page() to
mlock the page. This call is a no-op if the page is already locked,
but will mlock any pages in the non-linear mapping that happen to be
unlocked.
If one of the pages so mlocked is the page passed in to try_to_unmap(),
try_to_unmap_cluster() will return SWAP_MLOCK, rather than the default
SWAP_AGAIN. This will allow vmscan to cull the page, rather than
recirculating it on the inactive list.
Again, if try_to_unmap_cluster() cannot acquire the VMA's mmap sem, it
returns SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED
VMA, but couldn't be mlocked.
try_to_munlock() REVERSE MAP SCAN
---------------------------------
......@@ -625,10 +579,9 @@ introduced a variant of try_to_unmap() called try_to_munlock().
try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
mapped file pages with an additional argument specifying unlock versus unmap
processing. Again, these functions walk the respective reverse maps looking
for VM_LOCKED VMAs. When such a VMA is found for anonymous pages and file
pages mapped in linear VMAs, as in the try_to_unmap() case, the functions
attempt to acquire the associated mmap semaphore, mlock the page via
mlock_vma_page() and return SWAP_MLOCK. This effectively undoes the
for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case,
the functions attempt to acquire the associated mmap semaphore, mlock the page
via mlock_vma_page() and return SWAP_MLOCK. This effectively undoes the
pre-clearing of the page's PG_mlocked done by munlock_vma_page.
If try_to_unmap() is unable to acquire a VM_LOCKED VMA's associated mmap
......@@ -636,12 +589,6 @@ semaphore, it will return SWAP_AGAIN. This will allow shrink_page_list() to
recycle the page on the inactive list and hope that it has better luck with the
page next time.
For file pages mapped into non-linear VMAs, the try_to_munlock() logic works
slightly differently. On encountering a VM_LOCKED non-linear VMA that might
map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking the
page. munlock_vma_page() will just leave the page unlocked and let vmscan deal
with it - the usual fallback position.
Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
However, the scan can terminate when it encounters a VM_LOCKED VMA and can
......
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