Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit db6e3304 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge branch 'akpm' (patches from Andrew Morton) 

Merge more patches from Andrew Morton:
 "The rest of MM"

* emailed patches from Andrew Morton <akpm@linux-foundation.org>:
  mm: remove free_area_cache
  zswap: add documentation
  zswap: add to mm/
  zbud: add to mm/
parents ae924949 98d1e64f

Documentation/vm/zswap.txt

0 → 100644
+68 −0
Original line number Diff line number Diff line 
Overview:



Zswap is a lightweight compressed cache for swap pages. It takes pages that are

in the process of being swapped out and attempts to compress them into a

dynamically allocated RAM-based memory pool.  zswap basically trades CPU cycles

for potentially reduced swap I/O.  This trade-off can also result in a

significant performance improvement if reads from the compressed cache are

faster than reads from a swap device.



NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory

reclaim.  This interaction has not be fully explored on the large set of

potential configurations and workloads that exist.  For this reason, zswap

is a work in progress and should be considered experimental.



Some potential benefits:

* Desktop/laptop users with limited RAM capacities can mitigate the

    performance impact of swapping.

* Overcommitted guests that share a common I/O resource can

    dramatically reduce their swap I/O pressure, avoiding heavy handed I/O

    throttling by the hypervisor. This allows more work to get done with less

    impact to the guest workload and guests sharing the I/O subsystem

* Users with SSDs as swap devices can extend the life of the device by

    drastically reducing life-shortening writes.



Zswap evicts pages from compressed cache on an LRU basis to the backing swap

device when the compressed pool reaches it size limit.  This requirement had

been identified in prior community discussions.



To enabled zswap, the "enabled" attribute must be set to 1 at boot time.  e.g.

zswap.enabled=1



Design:



Zswap receives pages for compression through the Frontswap API and is able to

evict pages from its own compressed pool on an LRU basis and write them back to

the backing swap device in the case that the compressed pool is full.



Zswap makes use of zbud for the managing the compressed memory pool.  Each

allocation in zbud is not directly accessible by address.  Rather, a handle is

return by the allocation routine and that handle must be mapped before being

accessed.  The compressed memory pool grows on demand and shrinks as compressed

pages are freed.  The pool is not preallocated.



When a swap page is passed from frontswap to zswap, zswap maintains a mapping

of the swap entry, a combination of the swap type and swap offset, to the zbud

handle that references that compressed swap page.  This mapping is achieved

with a red-black tree per swap type.  The swap offset is the search key for the

tree nodes.



During a page fault on a PTE that is a swap entry, frontswap calls the zswap

load function to decompress the page into the page allocated by the page fault

handler.



Once there are no PTEs referencing a swap page stored in zswap (i.e. the count

in the swap_map goes to 0) the swap code calls the zswap invalidate function,

via frontswap, to free the compressed entry.



Zswap seeks to be simple in its policies.  Sysfs attributes allow for one user

controlled policies:

* max_pool_percent - The maximum percentage of memory that the compressed

    pool can occupy.



Zswap allows the compressor to be selected at kernel boot time by setting the

“compressor” attribute.  The default compressor is lzo.  e.g.

zswap.compressor=deflate



A debugfs interface is provided for various statistic about pool size, number

of pages stored, and various counters for the reasons pages are rejected.

arch/arm/mm/mmap.c

+0 −2
Original line number Diff line number Diff line
@@ -181,11 +181,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm) 
	if (mmap_is_legacy()) {

		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;

		mm->get_unmapped_area = arch_get_unmapped_area;

		mm->unmap_area = arch_unmap_area;

	} else {

		mm->mmap_base = mmap_base(random_factor);

		mm->get_unmapped_area = arch_get_unmapped_area_topdown;

		mm->unmap_area = arch_unmap_area_topdown;

	}

}

arch/arm64/mm/mmap.c

+0 −2
Original line number Diff line number Diff line
@@ -90,11 +90,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm) 
	if (mmap_is_legacy()) {

		mm->mmap_base = TASK_UNMAPPED_BASE;

		mm->get_unmapped_area = arch_get_unmapped_area;

		mm->unmap_area = arch_unmap_area;

	} else {

		mm->mmap_base = mmap_base();

		mm->get_unmapped_area = arch_get_unmapped_area_topdown;

		mm->unmap_area = arch_unmap_area_topdown;

	}

}

EXPORT_SYMBOL_GPL(arch_pick_mmap_layout);

arch/mips/mm/mmap.c

+0 −2
Original line number Diff line number Diff line
@@ -158,11 +158,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm) 
	if (mmap_is_legacy()) {

		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;

		mm->get_unmapped_area = arch_get_unmapped_area;

		mm->unmap_area = arch_unmap_area;

	} else {

		mm->mmap_base = mmap_base(random_factor);

		mm->get_unmapped_area = arch_get_unmapped_area_topdown;

		mm->unmap_area = arch_unmap_area_topdown;

	}

}

arch/powerpc/mm/mmap.c

+0 −2
Original line number Diff line number Diff line
@@ -92,10 +92,8 @@ void arch_pick_mmap_layout(struct mm_struct *mm) 
	if (mmap_is_legacy()) {

		mm->mmap_base = TASK_UNMAPPED_BASE;

		mm->get_unmapped_area = arch_get_unmapped_area;

		mm->unmap_area = arch_unmap_area;

	} else {

		mm->mmap_base = mmap_base();

		mm->get_unmapped_area = arch_get_unmapped_area_topdown;

		mm->unmap_area = arch_unmap_area_topdown;

	}

}