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Date: Tue, 20 Dec 2022 17:55:53 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, urezki@...il.com,
stephen.s.brennan@...cle.com, willy@...radead.org,
akpm@...ux-foundation.org, hch@...radead.org
Subject: Re: [PATCH v2 2/7] mm/vmalloc.c: add flags to mark vm_map_ram area
> Through vmalloc API, a virtual kernel area is reserved for physical
> address mapping. And vmap_area is used to track them, while vm_struct
> is allocated to associate with the vmap_area to store more information
> and passed out.
>
> However, area reserved via vm_map_ram() is an exception. It doesn't have
> vm_struct to associate with vmap_area. And we can't recognize the
> vmap_area with '->vm == NULL' as a vm_map_ram() area because the normal
> freeing path will set va->vm = NULL before unmapping, please see
> function remove_vm_area().
>
A normal "free" path sets it to NULL in order to prevent a double-free
of same VA. We can avoid of touching the va->vm if needed and do an unlink
on entry in the remove_vm_area() when a lock is taken to find an area.
Will it help you?
> Meanwhile, there are two types of vm_map_ram area. One is the whole
> vmap_area being reserved and mapped at one time; the other is the
> whole vmap_area with VMAP_BLOCK_SIZE size being reserved, while mapped
> into split regions with smaller size several times via vb_alloc().
>
> To mark the area reserved through vm_map_ram(), add flags field into
> struct vmap_area. Bit 0 indicates whether it's a vm_map_ram area,
> while bit 1 indicates whether it's a vmap_block type of vm_map_ram
> area.
>
> This is a preparatoin for later use.
>
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
> include/linux/vmalloc.h | 1 +
> mm/vmalloc.c | 22 +++++++++++++++++-----
> 2 files changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 096d48aa3437..69250efa03d1 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -76,6 +76,7 @@ struct vmap_area {
> unsigned long subtree_max_size; /* in "free" tree */
> struct vm_struct *vm; /* in "busy" tree */
> };
> + unsigned long flags; /* mark type of vm_map_ram area */
> };
>
> /* archs that select HAVE_ARCH_HUGE_VMAP should override one or more of these */
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 5d3fd3e6fe09..190f29bbaaa7 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1586,7 +1586,8 @@ preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)
> static struct vmap_area *alloc_vmap_area(unsigned long size,
> unsigned long align,
> unsigned long vstart, unsigned long vend,
> - int node, gfp_t gfp_mask)
> + int node, gfp_t gfp_mask,
> + unsigned long va_flags)
> {
> struct vmap_area *va;
> unsigned long freed;
> @@ -1630,6 +1631,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
> va->va_start = addr;
> va->va_end = addr + size;
> va->vm = NULL;
> + va->flags = va_flags;
>
> spin_lock(&vmap_area_lock);
> insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
> @@ -1887,6 +1889,10 @@ struct vmap_area *find_vmap_area(unsigned long addr)
>
> #define VMAP_BLOCK_SIZE (VMAP_BBMAP_BITS * PAGE_SIZE)
>
> +#define VMAP_RAM 0x1
> +#define VMAP_BLOCK 0x2
> +#define VMAP_FLAGS_MASK 0x3
>
Maybe to rename a VMAP_BLOCK to something like VMAP_BLOCK_RESERVED or
VMAP_PER_CPU_BLOCK?
> struct vmap_block_queue {
> spinlock_t lock;
> struct list_head free;
> @@ -1962,7 +1968,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>
> va = alloc_vmap_area(VMAP_BLOCK_SIZE, VMAP_BLOCK_SIZE,
> VMALLOC_START, VMALLOC_END,
> - node, gfp_mask);
> + node, gfp_mask,
> + VMAP_RAM|VMAP_BLOCK);
>
A new_vmap_block() is for a per-cpu path. As far as i see the VMAP_BLOCK
flag is used to mark a VA that corresponds to a reserved per-cpu free area.
Whereas a VMAP_RAM is for VA that was obtained over per-cpu path but
over alloc_vmap_area() thus a VA should be read out over "busy" tree
directly.
Why do you need to set here both VMAP_RAM and VMAP_BLOCK?
> if (IS_ERR(va)) {
> kfree(vb);
> return ERR_CAST(va);
> @@ -2229,8 +2236,12 @@ void vm_unmap_ram(const void *mem, unsigned int count)
> return;
> }
>
> - va = find_vmap_area(addr);
> + spin_lock(&vmap_area_lock);
> + va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
> BUG_ON(!va);
> + if (va)
> + va->flags &= ~VMAP_RAM;
> + spin_unlock(&vmap_area_lock);
> debug_check_no_locks_freed((void *)va->va_start,
>
Agree with Lorenzo. BUG_ON() should be out of spinlock(). Furthermore
i think it makes sense to go with WARN_ON_ONCE() and do not kill a system.
Instead emit a warning and bailout.
What do you think? Maybe separate patch for it?
> (va->va_end - va->va_start));
> free_unmap_vmap_area(va);
> @@ -2265,7 +2276,8 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
> } else {
> struct vmap_area *va;
> va = alloc_vmap_area(size, PAGE_SIZE,
> - VMALLOC_START, VMALLOC_END, node, GFP_KERNEL);
> + VMALLOC_START, VMALLOC_END,
> + node, GFP_KERNEL, VMAP_RAM);
> if (IS_ERR(va))
> return NULL;
>
> @@ -2505,7 +2517,7 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
> if (!(flags & VM_NO_GUARD))
> size += PAGE_SIZE;
>
> - va = alloc_vmap_area(size, align, start, end, node, gfp_mask);
> + va = alloc_vmap_area(size, align, start, end, node, gfp_mask, 0);
> if (IS_ERR(va)) {
> kfree(area);
> return NULL;
>
I know we have already discussed the new parameter. But what if we just
use atomic_set operation to mark VA as either vmap-ram or vmap-block?
As for alloc_vmap_area() we set it just as zero.
--
Uladzislau Rezki
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