man/man9/vm_page_alloc.9

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.Dd August 4, 2024
.Dt VM_PAGE_ALLOC 9
.Os
.Sh NAME
.Nm vm_page_alloc
.Nd "allocate a page of memory"
.Sh SYNOPSIS
.In sys/param.h
.In vm/vm.h
.In vm/vm_page.h
.Ft vm_page_t
.Fn vm_page_alloc "vm_object_t object" "vm_pindex_t pindex" "int req"
.Ft vm_page_t
.Fo vm_page_alloc_after
.Fa "vm_object_t object"
.Fa "vm_pindex_t pindex"
.Fa "int req"
.Fa "vm_page_t mpred"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_contig
.Fa "vm_object_t object"
.Fa "vm_pindex_t pindex"
.Fa "int req"
.Fa "u_long npages"
.Fa "vm_paddr_t low"
.Fa "vm_paddr_t high"
.Fa "u_long alignment"
.Fa "vm_paddr_t boundary"
.Fa "vm_memattr_t memattr"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_contig_domain
.Fa "vm_object_t object"
.Fa "vm_pindex_t pindex"
.Fa "int req"
.Fa "u_long npages"
.Fa "vm_paddr_t low"
.Fa "vm_paddr_t high"
.Fa "u_long alignment"
.Fa "vm_paddr_t boundary"
.Fa "vm_memattr_t memattr"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_domain
.Fa "vm_object_t object"
.Fa "vm_pindex_t pindex"
.Fa "int domain"
.Fa "int req"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_domain_after
.Fa "vm_object_t object"
.Fa "vm_pindex_t pindex"
.Fa "int domain"
.Fa "int req"
.Fa "vm_page_t mpred"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_noobj
.Fa "int req"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_noobj_contig
.Fa "int req"
.Fa "u_long npages"
.Fa "vm_paddr_t low"
.Fa "vm_paddr_t high"
.Fa "u_long alignment"
.Fa "vm_paddr_t boundary"
.Fa "vm_memattr_t memattr"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_noobj_contig_domain
.Fa "int domain"
.Fa "int req"
.Fa "u_long npages"
.Fa "vm_paddr_t low"
.Fa "vm_paddr_t high"
.Fa "u_long alignment"
.Fa "vm_paddr_t boundary"
.Fa "vm_memattr_t memattr"
.Fc
.Ft vm_page_t
.Fo vm_page_alloc_noobj_domain
.Fa "int domain"
.Fa "int req"
.Fc
.Sh DESCRIPTION
The
.Fn vm_page_alloc
family of functions allocate one or more pages of physical memory.
Most kernel code should not call these functions directly but should instead
use a kernel memory allocator such as
.Xr malloc 9
or
.Xr uma 9 ,
or should use a higher-level interface to the page cache, such as
.Xr vm_page_grab 9 .
.Pp
All of the functions take a
.Fa req
parameter which encodes the allocation priority and optional modifier flags,
described below.
The functions whose names do not include
.Dq noobj
additionally insert the pages starting at index
.Fa pindex
in the
VM object
.Fa object .
The object must be write-locked and not have a page already resident at the
specified index.
The functions whose names include
.Dq domain
support NUMA-aware allocation by returning pages from the
.Xr numa 4
domain specified by
.Fa domain .
.Pp
The
.Fn vm_page_alloc_after
and
.Fn vm_page_alloc_domain_after
functions behave identically to
.Fn vm_page_alloc
and
.Fn vm_page_alloc_domain ,
respectively, except that they take an additional parameter
.Fa mpred
which must be the page resident in
.Fa object
with largest index smaller than
.Fa pindex ,
or
.Dv NULL
if no such page exists.
These functions exist to optimize the common case of loops that allocate
multiple pages at successive indices within an object.
.Pp
The
.Fn vm_page_alloc_contig
and
.Fn vm_page_alloc_noobj_contig
functions and their NUMA-aware variants allocate a physically contiguous run of
.Fa npages
pages which satisfies the specified constraints.
The
.Fa low
and
.Fa high
parameters specify a physical address range from which the run is to
be allocated.
The
.Fa alignment
parameter specifies the requested alignment of the first page in the run
and must be a power of two.
If the
.Fa boundary
parameter is non-zero, the pages constituting the run will not cross a
physical address that is a multiple of the parameter value, which must be a
power of two.
If
.Fa memattr
is not equal to
.Dv VM_MEMATTR_DEFAULT ,
then mappings of the returned pages created by, e.g.,
.Xr pmap_enter 9
or
.Xr pmap_qenter 9 ,
will carry the machine-dependent encoding of the memory attribute.
Additionally, the direct mapping of the page, if any, will be updated to
reflect the requested memory attribute.
.Sh REQUEST FLAGS
All page allocator functions accept a
.Fa req
parameter that governs certain aspects of the function's behavior.
.Pp
The
.Dv VM_ALLOC_WAITOK ,
.Dv VM_ALLOC_WAITFAIL ,
and
.Dv VM_ALLOC_NOWAIT
flags specify the behavior of the allocator if free pages could not be
immediately allocated.
The
.Dv VM_ALLOC_WAITOK
flag can only be used with the
.Dq noobj
variants.
If
.Dv VM_ALLOC_NOWAIT
is specified, then the allocator gives up and returns
.Dv NULL .
.Dv VM_ALLOC_NOWAIT
is specified implicitly if none of the flags are present in the request.
If either
.Dv VM_ALLOC_WAITOK
or
.Dv VM_ALLOC_WAITFAIL
is specified, the allocator will put the calling thread to sleep until
sufficient free pages become available.
At this point, if
.Dv VM_ALLOC_WAITFAIL
is specified the allocator will return
.Dv NULL ,
and if
.Dv VM_ALLOC_WAITOK
is specified the allocator will retry the allocation.
After a failed
.Dv VM_ALLOC_WAITFAIL
allocation returns, the VM object, if any, will have been unlocked while the
thread was sleeping.
In this case the VM object write lock will be re-acquired before the function
call returns.
.Pp
.Fa req
also encodes the allocation request priority.
By default the page(s) are allocated with no special treatment.
If the number of available free pages is below a certain watermark, the
allocation will fail or the allocating thread will sleep, depending on
the specified wait flag.
The watermark is computed at boot time and corresponds to a small (less than
one percent) fraction of the system's total physical memory.
To allocate memory more aggressively, one of following flags may be specified.
.Bl -tag -width ".Dv VM_ALLOC_INTERRUPT"
.It Dv VM_ALLOC_SYSTEM
The page can be allocated if the free page count is above the interrupt
reserved water mark.
This flag should be used only when the system really needs the page.
.It Dv VM_ALLOC_INTERRUPT
The allocation will fail only if zero free pages are available.
This flag should be used only if the consequences of an allocation failure
are worse than leaving the system without free memory.
For example, this flag is used when allocating kernel page table pages, where
allocation failures trigger a kernel panic.
.El
.Pp
The following optional flags can further modify allocator behavior:
.Bl -tag -width ".Dv VM_ALLOC_NOBUSY"
.It Dv VM_ALLOC_SBUSY
The returned page will be shared-busy.
This flag may only be specified when allocating pages in a VM object.
.It Dv VM_ALLOC_NOBUSY
The returned page will not be busy.
This flag is implicit when allocating pages without a VM object.
When allocating pages in a VM object, and neither
.Dv VM_ALLOC_SBUSY
nor
.Dv VM_ALLOC_NOBUSY
are specified, the returned pages will be exclusively busied.
.It Dv VM_ALLOC_NODUMP
The returned page will not be included in any kernel core dumps
regardless of whether or not it is mapped in to KVA.
.It Dv VM_ALLOC_WIRED
The returned page will be wired.
.It Dv VM_ALLOC_ZERO
If this flag is specified, the
.Dq noobj
variants will return zeroed pages.
The other allocator interfaces ignore this flag.
.It Dv VM_ALLOC_NORECLAIM
If this flag is specified and the request can not be immediately satisfied,
the allocator will not attempt to break superpage reservations to satisfy the
allocation.
This may be useful when the overhead of scanning the reservation queue
outweighs the cost of a failed allocation.
This flag may be used only with the
.Dq contig
variants, and must not be specified in combination with
.Dv VM_ALLOC_WAITOK .
.It Dv VM_ALLOC_COUNT(n)
Hint that at least
.Fa n
pages will be allocated by the caller in the near future.
.Fa n
must be no larger than 65535.
If the system is short of free pages, this hint may cause the kernel
to reclaim memory more aggressively than it would otherwise.
.It Dv VM_ALLOC_NOFREE
The caller asserts that the returned page will never be released.
If this flag is specified, the allocator will try to fetch a page from a
special per-domain arena in order to curb long-term physical memory fragmentation.
.El
.Sh RETURN VALUES
If the allocation was successful, a pointer to the
.Vt struct vm_page
corresponding to the allocated page is returned.
If the allocation request specified multiple pages, the returned
pointer points to an array of
.Vt struct vm_page
constituting the run.
Upon failure,
.Dv NULL
is returned.
Regardless of whether the allocation succeeds or fails, the VM
object
.Fa object
will be write-locked upon return.
.Sh SEE ALSO
.Xr numa 4 ,
.Xr malloc 9 ,
.Xr uma 9 ,
.Xr vm_page_grab 9 ,
.Xr vm_page_sbusy 9
.Sh AUTHORS
This manual page was written by
.An Chad David Aq Mt davidc@acns.ab.ca .