NothingOSS/android_kernel_msm-6.1_nothing_sm7635
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UPSTREAM: binder: document the final page calculation

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The code to determine the page range for binder_lru_freelist_del() is
quite obscure. It leverages the buffer_size calculated before doing an
oversized buffer split. This is used to figure out if the last page is
being shared with another active buffer. If so, the page gets trimmed
out of the range as it has been previously removed from the freelist.

This would be equivalent to getting the start page of the next in-use
buffer explicitly. However, the code for this is much larger as we can
see in binder_free_buf_locked() routine. Instead, lets settle on
documenting the tricky step and using better names for now.

I believe an ideal solution would be to count the binder_page->users to
determine when a page should be added or removed from the freelist.
However, this is a much bigger change than what I'm willing to risk at
this time.

Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20231201172212.1813387-24-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 254650075
(cherry picked from commit 67dcc880780569ec40391cae4d8299adc1e7a44e)
Change-Id: Iec2466605fe7f8aa338c8313f586cdb7519a36e7
Signed-off-by: Carlos Llamas <cmllamas@google.com>
This commit is contained in:
Carlos Llamas 2023-12-01 17:21:52 +00:00
parent 19d966c1c6
commit baef4637fc
1 changed files with 11 additions and 7 deletions
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18
drivers/android/binder_alloc.c
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@ -446,8 +446,8 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
struct rb_node *n = alloc->free_buffers.rb_node;
struct rb_node *best_fit = NULL;
struct binder_buffer *buffer;
unsigned long has_page_addr;
unsigned long end_page_addr;
unsigned long next_used_page;
unsigned long curr_last_page;
size_t buffer_size;
if (is_async && alloc->free_async_space < size) {
@ -500,12 +500,16 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
"%d: binder_alloc_buf size %zd got buffer %pK size %zd\n",
alloc->pid, size, buffer, buffer_size);
has_page_addr = (buffer->user_data + buffer_size) & PAGE_MASK;
end_page_addr = PAGE_ALIGN(buffer->user_data + size);
if (end_page_addr > has_page_addr)
end_page_addr = has_page_addr;
/*
* Now we remove the pages from the freelist. A clever calculation
* with buffer_size determines if the last page is shared with an
* adjacent in-use buffer. In such case, the page has been already
* removed from the freelist so we trim our range short.
*/
next_used_page = (buffer->user_data + buffer_size) & PAGE_MASK;
curr_last_page = PAGE_ALIGN(buffer->user_data + size);
binder_lru_freelist_del(alloc, PAGE_ALIGN(buffer->user_data),
end_page_addr);
min(next_used_page, curr_last_page));
rb_erase(&buffer->rb_node, &alloc->free_buffers);
buffer->free = 0;