Date: Fri, 16 Feb 2024 08:44:58 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
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Subject: Re: [PATCH v3 14/35] lib: introduce support for page allocation tagging

On Fri, Feb 16, 2024 at 1:45 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 2/12/24 22:39, Suren Baghdasaryan wrote:
> > Introduce helper functions to easily instrument page allocators by
> > storing a pointer to the allocation tag associated with the code that
> > allocated the page in a page_ext field.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > Co-developed-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > +
> > +#ifdef CONFIG_MEM_ALLOC_PROFILING
> > +
> > +#include <linux/page_ext.h>
> > +
> > +extern struct page_ext_operations page_alloc_tagging_ops;
> > +extern struct page_ext *page_ext_get(struct page *page);
> > +extern void page_ext_put(struct page_ext *page_ext);
> > +
> > +static inline union codetag_ref *codetag_ref_from_page_ext(struct page_ext *page_ext)
> > +{
> > +     return (void *)page_ext + page_alloc_tagging_ops.offset;
> > +}
> > +
> > +static inline struct page_ext *page_ext_from_codetag_ref(union codetag_ref *ref)
> > +{
> > +     return (void *)ref - page_alloc_tagging_ops.offset;
> > +}
> > +
> > +static inline union codetag_ref *get_page_tag_ref(struct page *page)
> > +{
> > +     if (page && mem_alloc_profiling_enabled()) {
> > +             struct page_ext *page_ext = page_ext_get(page);
> > +
> > +             if (page_ext)
> > +                     return codetag_ref_from_page_ext(page_ext);
>
> I think when structured like this, you're not getting the full benefits of
> static keys, and the compiler probably can't improve that on its own.
>
> - page is tested before the static branch is evaluated
> - when disabled, the result is NULL, and that's again tested in the callers

Yes, that sounds right. I'll move the static branch check earlier like
you suggested. Thanks!

>
> > +     }
> > +     return NULL;
> > +}
> > +
> > +static inline void put_page_tag_ref(union codetag_ref *ref)
> > +{
> > +     page_ext_put(page_ext_from_codetag_ref(ref));
> > +}
> > +
> > +static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
> > +                                unsigned int order)
> > +{
> > +     union codetag_ref *ref = get_page_tag_ref(page);
>
> So the more optimal way would be to test mem_alloc_profiling_enabled() here
> as the very first thing before trying to get the ref.
>
> > +     if (ref) {
> > +             alloc_tag_add(ref, task->alloc_tag, PAGE_SIZE << order);
> > +             put_page_tag_ref(ref);
> > +     }
> > +}
> > +
> > +static inline void pgalloc_tag_sub(struct page *page, unsigned int order)
> > +{
> > +     union codetag_ref *ref = get_page_tag_ref(page);
>
> And same here.
>
> > +     if (ref) {
> > +             alloc_tag_sub(ref, PAGE_SIZE << order);
> > +             put_page_tag_ref(ref);
> > +     }
> > +}
> > +
> > +#else /* CONFIG_MEM_ALLOC_PROFILING */
> > +
> > +static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
> > +                                unsigned int order) {}
> > +static inline void pgalloc_tag_sub(struct page *page, unsigned int order) {}
> > +
> > +#endif /* CONFIG_MEM_ALLOC_PROFILING */
> > +
> > +#endif /* _LINUX_PGALLOC_TAG_H */
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 78d258ca508f..7bbdb0ddb011 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -978,6 +978,7 @@ config MEM_ALLOC_PROFILING
> >       depends on PROC_FS
> >       depends on !DEBUG_FORCE_WEAK_PER_CPU
> >       select CODE_TAGGING
> > +     select PAGE_EXTENSION
> >       help
> >         Track allocation source code and record total allocation size
> >         initiated at that code location. The mechanism can be used to track
> > diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> > index 4fc031f9cefd..2d5226d9262d 100644
> > --- a/lib/alloc_tag.c
> > +++ b/lib/alloc_tag.c
> > @@ -3,6 +3,7 @@
> >  #include <linux/fs.h>
> >  #include <linux/gfp.h>
> >  #include <linux/module.h>
> > +#include <linux/page_ext.h>
> >  #include <linux/proc_fs.h>
> >  #include <linux/seq_buf.h>
> >  #include <linux/seq_file.h>
> > @@ -124,6 +125,22 @@ static bool alloc_tag_module_unload(struct codetag_type *cttype,
> >       return module_unused;
> >  }
> >
> > +static __init bool need_page_alloc_tagging(void)
> > +{
> > +     return true;
>
> So this means the page_ext memory overead is paid unconditionally once
> MEM_ALLOC_PROFILING is compile time enabled, even if never enabled during
> runtime? That makes it rather costly to be suitable for generic distro
> kernels where the code could be compile time enabled, and runtime enabling
> suggested in a debugging/support scenario. It's what we do with page_owner,
> debug_pagealloc, slub_debug etc.
>
> Ideally we'd have some vmalloc based page_ext flavor for later-than-boot
> runtime enablement, as we now have for stackdepot. But that could be
> explored later. For now it would be sufficient to add an early_param boot
> parameter to control the enablement including page_ext, like page_owner and
> other features do.

Sounds reasonable. In v1 of this patchset we used early boot parameter
but after LSF/MM discussion that was changed to runtime controls.
Sounds like we would need both here. Should be easy to add.

Allocating/reclaiming dynamically the space for page_ext, slab_ext,
etc is not trivial and if done would be done separately. I looked into
it before and listed the encountered issues in the cover letter of v2
[1], see "things we could not address" section.

[1] https://lore.kernel.org/all/20231024134637.3120277-1-surenb@google.com/

>
> > +}
> > +
> > +static __init void init_page_alloc_tagging(void)
> > +{
> > +}
> > +
> > +struct page_ext_operations page_alloc_tagging_ops = {
> > +     .size = sizeof(union codetag_ref),
> > +     .need = need_page_alloc_tagging,
> > +     .init = init_page_alloc_tagging,
> > +};
> > +EXPORT_SYMBOL(page_alloc_tagging_ops);
>
>
> --
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