* User allocates ZFOD region, but does not actually touch the buffer
to fault in the pages.
* In a loop, user writes this buffer to a network socket, triggering
sosend_loan().
* uvm_loan() calls uvm_loanzero() once for each page in the loaned
region (since the pages have not yet faulted in). This causes a
page to be allocated and zero'd. The result is the kernel spends
a lot of time allocating and zero'ing pages.
This fixes creates a special object which owns a single zero'd page.
This single zero'd page is used to satisfy all loans of non-resident
ZFOD mappings.
Thanks to Allen Briggs for discovering the problem and for providing
an initial patch.
the page is still loaned to an anon, we should put the page back on a
paging queue. this is because while pages loaned to the kernel really
do need to stay resident (since the kernel is accessing the physical
memory directly), pages loaned to anons can be paged out just fine.
(the page will be paged out twice, first to the object and then again
to the anon, but after that the page can be reused.)
This makes `tail -<N> <FILE> | cat > file' correctly, where <FILE> is
a regular file larger than 10Mbytes (makes tail to map part of file)
and <N> is big enough to produce output larger than 8kbytes (makes pipe
to use page loan facility). Problem reported by FUKAUMI Naoki on japanese
local mailing list.
uobject and uanon pointers rather than at the PQ_ANON flag to determine
which lock to hold, since PQ_ANON can be clear even when the anon's lock
is the one which we should hold (if the page was loaned from an object
and then freed by the object).
- fix the loaned case in uvm_pagefree().
- redo uvmexp.swpgonly accounting to work with page loaning.
add an assertion before each place we adjust uvmexp.swpgonly.
- fix uvm_km_pgremove() to always free any swap space associated with
the range being removed.
- get rid of UVM_LOAN_WIRED flag. instead, we just make sure that
pages loaned to the kernel are never on the page queues.
this allows us to assert that pages are not loaned and wired
at the same time.
- add yet more assertions.
are only wired if this flag is present (i.e. they are not wired by default now)
loaned pages are unloaned via new uvm_unloan(), uvm_unloananon() and
uvm_unloanpage() are no longer exported
adjust uvm_unloanpage() to unwire the pages if UVM_LOAN_WIRED is specified
mark uvm_loanuobj() and uvm_loanzero() static also in function implementation
kern/sys_pipe.c: uvm_unloanpage() --> uvm_unloan()
- remove special treatment of pager_map mappings in pmaps. this is
required now, since I've removed the globals that expose the address range.
pager_map now uses pmap_kenter_pa() instead of pmap_enter(), so there's
no longer any need to special-case it.
- eliminate struct uvm_vnode by moving its fields into struct vnode.
- rewrite the pageout path. the pager is now responsible for handling the
high-level requests instead of only getting control after a bunch of work
has already been done on its behalf. this will allow us to UBCify LFS,
which needs tighter control over its pages than other filesystems do.
writing a page to disk no longer requires making it read-only, which
allows us to write wired pages without causing all kinds of havoc.
- use a new PG_PAGEOUT flag to indicate that a page should be freed
on behalf of the pagedaemon when it's unlocked. this flag is very similar
to PG_RELEASED, but unlike PG_RELEASED, PG_PAGEOUT can be cleared if the
pageout fails due to eg. an indirect-block buffer being locked.
this allows us to remove the "version" field from struct vm_page,
and together with shrinking "loan_count" from 32 bits to 16,
struct vm_page is now 4 bytes smaller.
- no longer use PG_RELEASED for swap-backed pages. if the page is busy
because it's being paged out, we can't release the swap slot to be
reallocated until that write is complete, but unlike with vnodes we
don't keep a count of in-progress writes so there's no good way to
know when the write is done. instead, when we need to free a busy
swap-backed page, just sleep until we can get it busy ourselves.
- implement a fast-path for extending writes which allows us to avoid
zeroing new pages. this substantially reduces cpu usage.
- encapsulate the data used by the genfs code in a struct genfs_node,
which must be the first element of the filesystem-specific vnode data
for filesystems which use genfs_{get,put}pages().
- eliminate many of the UVM pagerops, since they aren't needed anymore
now that the pager "put" operation is a higher-level operation.
- enhance the genfs code to allow NFS to use the genfs_{get,put}pages
instead of a modified copy.
- clean up struct vnode by removing all the fields that used to be used by
the vfs_cluster.c code (which we don't use anymore with UBC).
- remove kmem_object and mb_object since they were useless.
instead of allocating pages to these objects, we now just allocate
pages with no object. such pages are mapped in the kernel until they
are freed, so we can use the mapping to find the page to free it.
this allows us to remove splvm() protection in several places.
The sum of all these changes improves write throughput on my
decstation 5000/200 to within 1% of the rate of NetBSD 1.5
and reduces the elapsed time for "make release" of a NetBSD 1.5
source tree on my 128MB pc to 10% less than a 1.5 kernel took.
loop returns 0. loanentry was returning >0, but was unlocking the maps
(because of the zero). reworked to avoid this. problem reported by
chuck silvers. also clarify a comment that jdolecek asked about.
on locking rules to make code easier to understand. locking in
uvm_loananon still needs some work on fringe cases where anon's page
is actually on loan from a uobj.
if uvm_loanentry() returned 0; otherwise, the unlocking would already
have been done by uvmfault_unlockall() call in uvm_loanentry().
Okay'ed by Chuck Silvers
the mapping is:
VM_PAGER_OK 0
VM_PAGER_BAD <unused>
VM_PAGER_FAIL <unused>
VM_PAGER_PEND 0 (see below)
VM_PAGER_ERROR EIO
VM_PAGER_AGAIN EAGAIN
VM_PAGER_UNLOCK EBUSY
VM_PAGER_REFAULT ERESTART
for async i/o requests, it used to be possible for the request to
be convert to sync, and the pager would return VM_PAGER_OK or VM_PAGER_PEND
to indicate whether the caller should perform post-i/o cleanup.
this is no longer allowed; pagers must now return 0 to indicate that
the async i/o was successfully started, and the caller never needs to
worry about doing the post-i/o cleanup.
<vm/pglist.h> -> <uvm/uvm_pglist.h>
<vm/vm_inherit.h> -> <uvm/uvm_inherit.h>
<vm/vm_kern.h> -> into <uvm/uvm_extern.h>
<vm/vm_object.h> -> nothing
<vm/vm_pager.h> -> into <uvm/uvm_pager.h>
also includes a bunch of <vm/vm_page.h> include removals (due to redudancy
with <vm/vm.h>), and a scattering of other similar headers.
define a flag UVM_PGA_USERESERVE to allow non-kernel object
allocations to use pages from the reserve.
use the new flag for allocations in pmap modules.
- break anon related functions out of uvm_amap.c and put them in their own
file (uvm_anon.c). includes break up uvm_anon_init into an amap and an
an anon init function
- ensure that only functions within the amap module access amap structure
fields (add macros to amap api as needed)
UVM was written by chuck cranor <chuck@maria.wustl.edu>, with some
minor portions derived from the old Mach code. i provided some help
getting swap and paging working, and other bug fixes/ideas. chuck
silvers <chuq@chuq.com> also provided some other fixes.
this is the UVM kernel code portion.
this will be KNF'd shortly. :-)
yamt