From: Chengming Zhou <[email protected]>
We only need to save the position (pfn + obj_idx) in the handle, don't
need to save tag bit in handle. So one more bit can be used as obj_idx.
Actually, the tag bit is only useful in zspage's memory space, to tell
if an object is allocated or not.
Signed-off-by: Chengming Zhou <[email protected]>
---
mm/zsmalloc.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 63ec385cd670..7d7cb3eaabe0 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -110,7 +110,7 @@
#define OBJ_TAG_BITS 1
#define OBJ_TAG_MASK OBJ_ALLOCATED_TAG
-#define OBJ_INDEX_BITS (BITS_PER_LONG - _PFN_BITS - OBJ_TAG_BITS)
+#define OBJ_INDEX_BITS (BITS_PER_LONG - _PFN_BITS)
#define OBJ_INDEX_MASK ((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
#define HUGE_BITS 1
@@ -737,14 +737,12 @@ static struct page *get_next_page(struct page *page)
static void obj_to_location(unsigned long obj, struct page **page,
unsigned int *obj_idx)
{
- obj >>= OBJ_TAG_BITS;
*page = pfn_to_page(obj >> OBJ_INDEX_BITS);
*obj_idx = (obj & OBJ_INDEX_MASK);
}
static void obj_to_page(unsigned long obj, struct page **page)
{
- obj >>= OBJ_TAG_BITS;
*page = pfn_to_page(obj >> OBJ_INDEX_BITS);
}
@@ -759,7 +757,6 @@ static unsigned long location_to_obj(struct page *page, unsigned int obj_idx)
obj = page_to_pfn(page) << OBJ_INDEX_BITS;
obj |= obj_idx & OBJ_INDEX_MASK;
- obj <<= OBJ_TAG_BITS;
return obj;
}
--
2.40.1
On (24/02/27 03:00), [email protected] wrote:
>
> We only need to save the position (pfn + obj_idx) in the handle, don't
> need to save tag bit in handle. So one more bit can be used as obj_idx.
[..]
> mm/zsmalloc: don't need to save tag bit in handle
Does this mean "don't need to reserve LSB for tag"?
We still save allocated tag in the handle, that's what
handle |= OBJ_ALLOCATED_TAG;
does.
> Actually, the tag bit is only useful in zspage's memory space, to tell
> if an object is allocated or not.
I'm not completely sure if I follow this sentence.
On 2024/2/27 15:52, Sergey Senozhatsky wrote:
> On (24/02/27 03:00), [email protected] wrote:
>>
>> We only need to save the position (pfn + obj_idx) in the handle, don't
>> need to save tag bit in handle. So one more bit can be used as obj_idx.
>
> [..]
>
>> mm/zsmalloc: don't need to save tag bit in handle
>
> Does this mean "don't need to reserve LSB for tag"?
The head of object still need to reverve LSB, to save (handle | OBJ_ALLOCATED_TAG),
only the handle doesn't need to reserve LSB, which save (pfn | obj_idx).
> We still save allocated tag in the handle, that's what
>
> handle |= OBJ_ALLOCATED_TAG;
Yes, this result will be saved in the head of each allocated object.
>
> does.
>
>> Actually, the tag bit is only useful in zspage's memory space, to tell
>> if an object is allocated or not.
>
> I'm not completely sure if I follow this sentence.
What I mean is that only the head of each allocated object need to reverve LSB,
which is used to check if allocated or not.
handle address -> handle (pfn + obj_idx) -> object: (handle | tag), real_object start
I'm not sure if this makes it clearer?
Thanks.
On 2024/2/28 09:54, Sergey Senozhatsky wrote:
> On (24/02/27 16:16), Chengming Zhou wrote:
>> On 2024/2/27 15:52, Sergey Senozhatsky wrote:
>>> On (24/02/27 03:00), [email protected] wrote:
>>>>
>>>> We only need to save the position (pfn + obj_idx) in the handle, don't
>>>> need to save tag bit in handle. So one more bit can be used as obj_idx.
>>>
>>> [..]
>>>
>>>> mm/zsmalloc: don't need to save tag bit in handle
>>>
>>> Does this mean "don't need to reserve LSB for tag"?
>> The head of object still need to reverve LSB, to save (handle | OBJ_ALLOCATED_TAG),
>> only the handle doesn't need to reserve LSB, which save (pfn | obj_idx).
>
> Correct.
>
>>> We still save allocated tag in the handle, that's what
>>>
>>> handle |= OBJ_ALLOCATED_TAG;
>>
>> Yes, this result will be saved in the head of each allocated object.
>
> Right, that's what I was talking about.
>
>>>> Actually, the tag bit is only useful in zspage's memory space, to tell
>>>> if an object is allocated or not.
>>>
>>> I'm not completely sure if I follow this sentence.
>>
>> What I mean is that only the head of each allocated object need to reverve LSB,
>> which is used to check if allocated or not.
>>
>> handle address -> handle (pfn + obj_idx) -> object: (handle | tag), real_object start
>>
>> I'm not sure if this makes it clearer?
>
> Yes, thanks. I think separating handle and object header in the commit
> message will be helpful.
Right, I will improve the commit message and send v2.
Thanks!
On (24/02/27 16:16), Chengming Zhou wrote:
> On 2024/2/27 15:52, Sergey Senozhatsky wrote:
> > On (24/02/27 03:00), [email protected] wrote:
> >>
> >> We only need to save the position (pfn + obj_idx) in the handle, don't
> >> need to save tag bit in handle. So one more bit can be used as obj_idx.
> >
> > [..]
> >
> >> mm/zsmalloc: don't need to save tag bit in handle
> >
> > Does this mean "don't need to reserve LSB for tag"?
> The head of object still need to reverve LSB, to save (handle | OBJ_ALLOCATED_TAG),
> only the handle doesn't need to reserve LSB, which save (pfn | obj_idx).
Correct.
> > We still save allocated tag in the handle, that's what
> >
> > handle |= OBJ_ALLOCATED_TAG;
>
> Yes, this result will be saved in the head of each allocated object.
Right, that's what I was talking about.
> >> Actually, the tag bit is only useful in zspage's memory space, to tell
> >> if an object is allocated or not.
> >
> > I'm not completely sure if I follow this sentence.
>
> What I mean is that only the head of each allocated object need to reverve LSB,
> which is used to check if allocated or not.
>
> handle address -> handle (pfn + obj_idx) -> object: (handle | tag), real_object start
>
> I'm not sure if this makes it clearer?
Yes, thanks. I think separating handle and object header in the commit
message will be helpful.