Re: [PATCH v4] proc/ksm: add ksm stats to /proc/pid/smaps

From: Stefan Roesch
Date: Wed Aug 23 2023 - 12:43:45 EST


David Hildenbrand <david@xxxxxxxxxx> writes:

> On 22.08.23 20:05, Stefan Roesch wrote:
>> With madvise and prctl KSM can be enabled for different VMA's. Once it
>> is enabled we can query how effective KSM is overall. However we cannot
>> easily query if an individual VMA benefits from KSM.
>> This commit adds a KSM section to the /prod/<pid>/smaps file. It reports
>> how many of the pages are KSM pages. The returned value for KSM is
>> independent of the use of the shared zeropage.
>
> Maybe phrase that to something like "The returned value for KSM includes
> KSM-placed zeropages, so we can observe the actual KSM benefit independent
> of the usage of the shared zeropage for KSM.".
>
> But thinking about it (see below), maybe we really just let any user figure that out by
> temporarily disabling the shared zeropage.
>
> So this would be
>
> "It reports how many of the pages are KSM pages. Note that KSM-placed zeropages
> are not included, only actual KSM pages."
>

I'll replace the commit message and the documentation with the above
sentence.

>> Here is a typical output:
>> 7f420a000000-7f421a000000 rw-p 00000000 00:00 0
>> Size: 262144 kB
>> KernelPageSize: 4 kB
>> MMUPageSize: 4 kB
>> Rss: 51212 kB
>> Pss: 8276 kB
>> Shared_Clean: 172 kB
>> Shared_Dirty: 42996 kB
>> Private_Clean: 196 kB
>> Private_Dirty: 7848 kB
>> Referenced: 15388 kB
>> Anonymous: 51212 kB
>> KSM: 41376 kB
>> LazyFree: 0 kB
>> AnonHugePages: 0 kB
>> ShmemPmdMapped: 0 kB
>> FilePmdMapped: 0 kB
>> Shared_Hugetlb: 0 kB
>> Private_Hugetlb: 0 kB
>> Swap: 202016 kB
>> SwapPss: 3882 kB
>> Locked: 0 kB
>> THPeligible: 0
>> ProtectionKey: 0
>> ksm_state: 0
>> ksm_skip_base: 0
>> ksm_skip_count: 0
>> VmFlags: rd wr mr mw me nr mg anon
>> This information also helps with the following workflow:
>> - First enable KSM for all the VMA's of a process with prctl.
>> - Then analyze with the above smaps report which VMA's benefit the most
>> - Change the application (if possible) to add the corresponding madvise
>> calls for the VMA's that benefit the most
>> Signed-off-by: Stefan Roesch <shr@xxxxxxxxxxxx>
>> ---
>> Documentation/filesystems/proc.rst | 4 ++++
>> fs/proc/task_mmu.c | 16 +++++++++++-----
>> 2 files changed, 15 insertions(+), 5 deletions(-)
>> diff --git a/Documentation/filesystems/proc.rst
>> b/Documentation/filesystems/proc.rst
>> index 7897a7dafcbc..d5bdfd59f5b0 100644
>> --- a/Documentation/filesystems/proc.rst
>> +++ b/Documentation/filesystems/proc.rst
>> @@ -461,6 +461,7 @@ Memory Area, or VMA) there is a series of lines such as the following::
>> Private_Dirty: 0 kB
>> Referenced: 892 kB
>> Anonymous: 0 kB
>> + KSM: 0 kB
>> LazyFree: 0 kB
>> AnonHugePages: 0 kB
>> ShmemPmdMapped: 0 kB
>> @@ -501,6 +502,9 @@ accessed.
>> a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
>> and a page is modified, the file page is replaced by a private anonymous copy.
>> +"KSM" shows the amount of anonymous memory that has been de-duplicated. The
>> +value is independent of the use of shared zeropage.
>
> Maybe here as well.
>
>> +
>> "LazyFree" shows the amount of memory which is marked by madvise(MADV_FREE).
>> The memory isn't freed immediately with madvise(). It's freed in memory
>> pressure if the memory is clean. Please note that the printed value might
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 51315133cdc2..4532caa8011c 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -4,6 +4,7 @@
>> #include <linux/hugetlb.h>
>> #include <linux/huge_mm.h>
>> #include <linux/mount.h>
>> +#include <linux/ksm.h>
>> #include <linux/seq_file.h>
>> #include <linux/highmem.h>
>> #include <linux/ptrace.h>
>> @@ -396,6 +397,7 @@ struct mem_size_stats {
>> unsigned long swap;
>> unsigned long shared_hugetlb;
>> unsigned long private_hugetlb;
>> + unsigned long ksm;
>> u64 pss;
>> u64 pss_anon;
>> u64 pss_file;
>> @@ -435,9 +437,9 @@ static void smaps_page_accumulate(struct mem_size_stats *mss,
>> }
>> }
>> -static void smaps_account(struct mem_size_stats *mss, struct page *page,
>> - bool compound, bool young, bool dirty, bool locked,
>> - bool migration)
>> +static void smaps_account(struct mem_size_stats *mss, pte_t *pte,
>> + struct page *page, bool compound, bool young, bool dirty,
>> + bool locked, bool migration)
>> {
>> int i, nr = compound ? compound_nr(page) : 1;
>> unsigned long size = nr * PAGE_SIZE;
>> @@ -452,6 +454,9 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>> mss->lazyfree += size;
>> }
>> + if (PageKsm(page) && (!pte || !is_ksm_zero_pte(*pte)))
>
> I think this won't work either way, because smaps_pte_entry() never ends up calling
> this function with !page. And the shared zeropage here always gives us !page.
>
> What would work is:
>
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 15ddf4653a19..ef6f39d7c5a2 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -528,6 +528,9 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
> page = vm_normal_page(vma, addr, ptent);
> young = pte_young(ptent);
> dirty = pte_dirty(ptent);
> +
> + if (!page && is_ksm_zero_pte(ptent))
> + mss->ksm += size;
> } else if (is_swap_pte(ptent)) {
> swp_entry_t swpent = pte_to_swp_entry(ptent);
> That means that "KSM" can be bigger than "Anonymous" and "RSS" when the shared
> zeropage is used.
>
> Interestingly, right now we account each KSM page individually towards
> "Anonymous" and "RSS".
>
> So if we have 100 times the same KSM page in a VMA, we will have 100 times anon
> and 100 times rss.
>
> Thinking about it, I guess considering the KSM-placed zeropage indeed adds more
> confusion to that. Eventually, we might just want separate "Shared-zeropages" count.
>
>
> So maybe v3 is better, clarifying the documentation a bit, that the
> KSM-placed zeropage is not considered.
>
o
> Sorry for changing my mind :D Thoughts?

I agree I think v3 is better, I'll revert to v3 and add the above
documentation change.