On 22/09/2020 15:31, Christoph Hellwig wrote:
On Tue, Sep 22, 2020 at 09:23:59AM +0100, Tvrtko Ursulin wrote:
If I understood this sub-thread correctly, iterating and freeing the pages
via the vmapped ptes, so no need for a
shmem_read_mapping_page_gfp loop in shmem_unpin_map looks plausible to me.
I did not get the reference to kernel/dma/remap.c though,
What I mean is the code in dma_common_find_pages, which returns the
page array for freeing.
Got it.
and also not sure
how to do the error unwind path in shmem_pin_map at which point the
allocated vm area hasn't been fully populated yet. Hand-roll the loop
walking vm area struct in there?
Yes. What I originally did (re-created as I didn't save it) would be
something like this:
---
From 5605e77cda246df6dd7ded99ec22cb3f341ef5d5 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@xxxxxx>
Date: Wed, 16 Sep 2020 13:54:04 +0200
Subject: drm/i915: use vmap in shmem_pin_map
shmem_pin_map somewhat awkwardly reimplements vmap using
alloc_vm_area and manual pte setup. The only practical difference
is that alloc_vm_area prefeaults the vmalloc area PTEs, which doesn't
seem to be required here (and could be added to vmap using a flag
if actually required).
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
drivers/gpu/drm/i915/gt/shmem_utils.c | 81 +++++++++------------------
1 file changed, 27 insertions(+), 54 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/shmem_utils.c b/drivers/gpu/drm/i915/gt/shmem_utils.c
index 43c7acbdc79dea..7ec6ba4c1065b2 100644
--- a/drivers/gpu/drm/i915/gt/shmem_utils.c
+++ b/drivers/gpu/drm/i915/gt/shmem_utils.c
@@ -49,80 +49,53 @@ struct file *shmem_create_from_object(struct drm_i915_gem_object *obj)
return file;
}
-static size_t shmem_npte(struct file *file)
+static size_t shmem_npages(struct file *file)
{
return file->f_mapping->host->i_size >> PAGE_SHIFT;
}
-static void __shmem_unpin_map(struct file *file, void *ptr, size_t n_pte)
-{
- unsigned long pfn;
-
- vunmap(ptr);
-
- for (pfn = 0; pfn < n_pte; pfn++) {
- struct page *page;
-
- page = shmem_read_mapping_page_gfp(file->f_mapping, pfn,
- GFP_KERNEL);
- if (!WARN_ON(IS_ERR(page))) {
- put_page(page);
- put_page(page);
- }
- }
-}
-
void *shmem_pin_map(struct file *file)
{
- const size_t n_pte = shmem_npte(file);
- pte_t *stack[32], **ptes, **mem;
Chris can comment how much he'd miss the 32 page stack shortcut.
- struct vm_struct *area;
- unsigned long pfn;
-
- mem = stack;
- if (n_pte > ARRAY_SIZE(stack)) {
- mem = kvmalloc_array(n_pte, sizeof(*mem), GFP_KERNEL);
- if (!mem)
- return NULL;
- }
+ size_t n_pages = shmem_npages(file), i;
+ struct page **pages;
+ void *vaddr;
- area = alloc_vm_area(n_pte << PAGE_SHIFT, mem);
- if (!area) {
- if (mem != stack)
- kvfree(mem);
+ pages = kvmalloc_array(n_pages, sizeof(*pages), GFP_KERNEL);
+ if (!pages)
return NULL;
- }
-
- ptes = mem;
- for (pfn = 0; pfn < n_pte; pfn++) {
- struct page *page;
- page = shmem_read_mapping_page_gfp(file->f_mapping, pfn,
- GFP_KERNEL);
- if (IS_ERR(page))
+ for (i = 0; i < n_pages; i++) {
+ pages[i] = shmem_read_mapping_page_gfp(file->f_mapping, i,
+ GFP_KERNEL);
+ if (IS_ERR(pages[i]))
goto err_page;
-
- **ptes++ = mk_pte(page, PAGE_KERNEL);
}
- if (mem != stack)
- kvfree(mem);
-
+ vaddr = vmap(pages, n_pages, 0, PAGE_KERNEL);
+ if (!vaddr)
+ goto err_page;
mapping_set_unevictable(file->f_mapping);
- return area->addr;
-
+ return vaddr;
Is there something in vmap() preventing us from freeing the pages array
here? I can't spot anything that is holding on to the pointer. Or it was
just a sketch before you realized we could walk the vm_area?
Also, I may be totally misunderstanding something, but I think you need
to assign area->pages manually so shmem_unpin_map can access it below.
err_page:
- if (mem != stack)
- kvfree(mem);
-
- __shmem_unpin_map(file, area->addr, pfn);
+ while (--i >= 0)
+ put_page(pages[i]);
+ kvfree(pages);
return NULL;
}
void shmem_unpin_map(struct file *file, void *ptr)
{
+ struct vm_struct *area = find_vm_area(ptr);
+ size_t i = shmem_npages(file);
+
+ if (WARN_ON_ONCE(!area || !area->pages))
+ return;
+
mapping_clear_unevictable(file->f_mapping);
- __shmem_unpin_map(file, ptr, shmem_npte(file));
+ for (i = 0; i < shmem_npages(file); i++)
+ put_page(area->pages[i]);
+ kvfree(area->pages);
+ vunmap(ptr);
Is the verdict from mm experts that we can't use vfree due __free_pages
vs put_page differences?
Could we get from ptes to pages, so that we don't have to keep the
area->pages array allocated for the duration of the pin?
Regards,
Tvrtko
}
static int __shmem_rw(struct file *file, loff_t off,
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