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 <Y8qm69bYnzZk+Vni@bfoster>
In-Reply-To: <Y8qm69bYnzZk+Vni@bfoster>
From:   Nhat Pham <nphamcs@gmail.com>
Date:   Fri, 20 Jan 2023 10:26:54 -0800
Message-ID: <CAKEwX=Pse=f=9d6mHM6vZ3PvT4=0UfUiA8eK2_T0vcgu3Wwbkg@mail.gmail.com>
Subject: Re: [PATCH v6 2/3] cachestat: implement cachestat syscall
To:     Brian Foster <bfoster@redhat.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, willy@infradead.org,
        linux-api@vger.kernel.org, kernel-team@meta.com
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On Fri, Jan 20, 2023 at 6:35 AM Brian Foster <bfoster@redhat.com> wrote:
>
> On Tue, Jan 17, 2023 at 11:59:58AM -0800, Nhat Pham wrote:
> > There is currently no good way to query the page cache state of large
> > file sets and directory trees. There is mincore(), but it scales poorly:
> > the kernel writes out a lot of bitmap data that userspace has to
> > aggregate, when the user really doesn not care about per-page
> > information in that case. The user also needs to mmap and unmap each
> > file as it goes along, which can be quite slow as well.
> >
> > This patch implements a new syscall that queries cache state of a file
> > and summarizes the number of cached pages, number of dirty pages, number
> > of pages marked for writeback, number of (recently) evicted pages, etc.
> > in a given range.
> >
> > NAME
> >     cachestat - query the page cache statistics of a file.
> >
> > SYNOPSIS
> >     #include <sys/mman.h>
> >
> >     struct cachestat {
> >         __u64 nr_cache;
> >         __u64 nr_dirty;
> >         __u64 nr_writeback;
> >         __u64 nr_evicted;
> >         __u64 nr_recently_evicted;
> >     };
> >
> >     int cachestat(unsigned int fd, off_t off, size_t len,
> >           unsigned int cstat_version, struct cachestat *cstat,
> >           unsigned int flags);
> >
> > DESCRIPTION
> >     cachestat() queries the number of cached pages, number of dirty
> >     pages, number of pages marked for writeback, number of evicted
> >     pages, number of recently evicted pages, in the bytes range given by
> >     `off` and `len`.
> >
> >     An evicted page is a page that is previously in the page cache but
> >     has been evicted since. A page is recently evicted if its last
> >     eviction was recent enough that its reentry to the cache would
> >     indicate that it is actively being used by the system, and that
> >     there is memory pressure on the system.
> >
> >     These values are returned in a cachestat struct, whose address is
> >     given by the `cstat` argument.
> >
> >     The `off` and `len` arguments must be non-negative integers. If
> >     `len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
> >     0, we will query in the range from `off` to the end of the file.
> >
> >     `cstat_version` is an unsigned integer indicating the specific
> >     version of the cachestat struct. It must be at least 1, and does
> >     not exceed the latest version number (which is currently 1). For
> >     now, user should just pass 1.
> >
>
> Still not sure about this vs. just requiring the structure size to match
> (and maybe padding it), but perhaps linux-api can comment on the best
> way to future proof.
>
> I'm not the person to ack a syscall regardless, but since I've had
> various previous comments on this patch and no further issues stand out
> to me:
>
> Reviewed-by: Brian Foster <bfoster@redhat.com>
Thanks for the review and feedback, Brian!
>
> >     The `flags` argument is unused for now, but is included for future
> >     extensibility. User should pass 0 (i.e no flag specified).
> >
> > RETURN VALUE
> >     On success, cachestat returns 0. On error, -1 is returned, and errno
> >     is set to indicate the error.
> >
> > ERRORS
> >     EFAULT cstat points to an invalid address.
> >
> >     EINVAL invalid `cstat_version` or `flags`
> >
> >     EBADF  invalid file descriptor.
> >
> > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > ---
> >  arch/alpha/kernel/syscalls/syscall.tbl      |   1 +
> >  arch/arm/tools/syscall.tbl                  |   1 +
> >  arch/ia64/kernel/syscalls/syscall.tbl       |   1 +
> >  arch/m68k/kernel/syscalls/syscall.tbl       |   1 +
> >  arch/microblaze/kernel/syscalls/syscall.tbl |   1 +
> >  arch/parisc/kernel/syscalls/syscall.tbl     |   1 +
> >  arch/powerpc/kernel/syscalls/syscall.tbl    |   1 +
> >  arch/s390/kernel/syscalls/syscall.tbl       |   1 +
> >  arch/sh/kernel/syscalls/syscall.tbl         |   1 +
> >  arch/sparc/kernel/syscalls/syscall.tbl      |   1 +
> >  arch/x86/entry/syscalls/syscall_32.tbl      |   1 +
> >  arch/x86/entry/syscalls/syscall_64.tbl      |   1 +
> >  arch/xtensa/kernel/syscalls/syscall.tbl     |   1 +
> >  include/linux/fs.h                          |   3 +
> >  include/linux/syscalls.h                    |   4 +
> >  include/uapi/asm-generic/unistd.h           |   5 +-
> >  include/uapi/linux/mman.h                   |   9 ++
> >  init/Kconfig                                |  10 ++
> >  kernel/sys_ni.c                             |   1 +
> >  mm/filemap.c                                | 154 ++++++++++++++++++++
> >  20 files changed, 198 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> > index 8ebacf37a8cf..1f13995d00d7 100644
> > --- a/arch/alpha/kernel/syscalls/syscall.tbl
> > +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> > @@ -490,3 +490,4 @@
> >  558  common  process_mrelease                sys_process_mrelease
> >  559  common  futex_waitv                     sys_futex_waitv
> >  560  common  set_mempolicy_home_node         sys_ni_syscall
> > +561  common  cachestat                       sys_cachestat
> > diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> > index ac964612d8b0..8ebed8a13874 100644
> > --- a/arch/arm/tools/syscall.tbl
> > +++ b/arch/arm/tools/syscall.tbl
> > @@ -464,3 +464,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
> > index 72c929d9902b..f8c74ffeeefb 100644
> > --- a/arch/ia64/kernel/syscalls/syscall.tbl
> > +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> > @@ -371,3 +371,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> > index b1f3940bc298..4f504783371f 100644
> > --- a/arch/m68k/kernel/syscalls/syscall.tbl
> > +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> > @@ -450,3 +450,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> > index 820145e47350..858d22bf275c 100644
> > --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> > +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> > @@ -456,3 +456,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> > index 8a99c998da9b..7c84a72306d1 100644
> > --- a/arch/parisc/kernel/syscalls/syscall.tbl
> > +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> > @@ -448,3 +448,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> > index 2bca64f96164..937460f0a8ec 100644
> > --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> > +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> > @@ -530,3 +530,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  nospu   set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> > index 799147658dee..7df0329d46cb 100644
> > --- a/arch/s390/kernel/syscalls/syscall.tbl
> > +++ b/arch/s390/kernel/syscalls/syscall.tbl
> > @@ -453,3 +453,4 @@
> >  448  common  process_mrelease        sys_process_mrelease            sys_process_mrelease
> >  449  common  futex_waitv             sys_futex_waitv                 sys_futex_waitv
> >  450  common  set_mempolicy_home_node sys_set_mempolicy_home_node     sys_set_mempolicy_home_node
> > +451  common  cachestat               sys_cachestat                   sys_cachestat
> > diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> > index 2de85c977f54..97377e8c5025 100644
> > --- a/arch/sh/kernel/syscalls/syscall.tbl
> > +++ b/arch/sh/kernel/syscalls/syscall.tbl
> > @@ -453,3 +453,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> > index 4398cc6fb68d..faa835f3c54a 100644
> > --- a/arch/sparc/kernel/syscalls/syscall.tbl
> > +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> > @@ -496,3 +496,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> > index 320480a8db4f..bc0a3c941b35 100644
> > --- a/arch/x86/entry/syscalls/syscall_32.tbl
> > +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> > @@ -455,3 +455,4 @@
> >  448  i386    process_mrelease        sys_process_mrelease
> >  449  i386    futex_waitv             sys_futex_waitv
> >  450  i386    set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  i386    cachestat               sys_cachestat
> > diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> > index c84d12608cd2..227538b0ce80 100644
> > --- a/arch/x86/entry/syscalls/syscall_64.tbl
> > +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> > @@ -372,6 +372,7 @@
> >  448  common  process_mrelease        sys_process_mrelease
> >  449  common  futex_waitv             sys_futex_waitv
> >  450  common  set_mempolicy_home_node sys_set_mempolicy_home_node
> > +451  common  cachestat               sys_cachestat
> >
> >  #
> >  # Due to a historical design error, certain syscalls are numbered differently
> > diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> > index 52c94ab5c205..2b69c3c035b6 100644
> > --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> > +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> > @@ -421,3 +421,4 @@
> >  448  common  process_mrelease                sys_process_mrelease
> >  449  common  futex_waitv                     sys_futex_waitv
> >  450  common  set_mempolicy_home_node         sys_set_mempolicy_home_node
> > +451  common  cachestat                       sys_cachestat
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index e654435f1651..83300f1491e7 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -75,6 +75,7 @@ struct fs_context;
> >  struct fs_parameter_spec;
> >  struct fileattr;
> >  struct iomap_ops;
> > +struct cachestat;
> >
> >  extern void __init inode_init(void);
> >  extern void __init inode_init_early(void);
> > @@ -830,6 +831,8 @@ void filemap_invalidate_lock_two(struct address_space *mapping1,
> >                                struct address_space *mapping2);
> >  void filemap_invalidate_unlock_two(struct address_space *mapping1,
> >                                  struct address_space *mapping2);
> > +void filemap_cachestat(struct address_space *mapping, pgoff_t first_index,
> > +             pgoff_t last_index, struct cachestat *cs);
> >
> >
> >  /*
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index a34b0f9a9972..d3fe6ba8eb38 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -72,6 +72,7 @@ struct open_how;
> >  struct mount_attr;
> >  struct landlock_ruleset_attr;
> >  enum landlock_rule_type;
> > +struct cachestat;
> >
> >  #include <linux/types.h>
> >  #include <linux/aio_abi.h>
> > @@ -1056,6 +1057,9 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> >  asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> >                                           unsigned long home_node,
> >                                           unsigned long flags);
> > +asmlinkage long sys_cachestat(unsigned int fd, off_t off, size_t len,
> > +             unsigned int cstat_version, struct cachestat __user *cstat,
> > +             unsigned int flags);
> >
> >  /*
> >   * Architecture-specific system calls
> > diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> > index 45fa180cc56a..cd639fae9086 100644
> > --- a/include/uapi/asm-generic/unistd.h
> > +++ b/include/uapi/asm-generic/unistd.h
> > @@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
> >  #define __NR_set_mempolicy_home_node 450
> >  __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
> >
> > +#define __NR_cachestat 451
> > +__SYSCALL(__NR_cachestat, sys_cachestat)
> > +
> >  #undef __NR_syscalls
> > -#define __NR_syscalls 451
> > +#define __NR_syscalls 452
> >
> >  /*
> >   * 32 bit systems traditionally used different
> > diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
> > index f55bc680b5b0..fe03ed0b7587 100644
> > --- a/include/uapi/linux/mman.h
> > +++ b/include/uapi/linux/mman.h
> > @@ -4,6 +4,7 @@
> >
> >  #include <asm/mman.h>
> >  #include <asm-generic/hugetlb_encode.h>
> > +#include <linux/types.h>
> >
> >  #define MREMAP_MAYMOVE               1
> >  #define MREMAP_FIXED         2
> > @@ -41,4 +42,12 @@
> >  #define MAP_HUGE_2GB HUGETLB_FLAG_ENCODE_2GB
> >  #define MAP_HUGE_16GB        HUGETLB_FLAG_ENCODE_16GB
> >
> > +struct cachestat {
> > +     __u64 nr_cache;
> > +     __u64 nr_dirty;
> > +     __u64 nr_writeback;
> > +     __u64 nr_evicted;
> > +     __u64 nr_recently_evicted;
> > +};
> > +
> >  #endif /* _UAPI_LINUX_MMAN_H */
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 694f7c160c9c..da96ac29af1d 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1798,6 +1798,16 @@ config RSEQ
> >
> >         If unsure, say Y.
> >
> > +config CACHESTAT_SYSCALL
> > +     bool "Enable cachestat() system call" if EXPERT
> > +     default y
> > +     help
> > +       Enable the cachestat system call, which queries the page cache
> > +       statistics of a file (number of cached pages, dirty pages,
> > +       pages marked for writeback, (recently) evicted pages).
> > +
> > +       If unsure say Y here.
> > +
> >  config DEBUG_RSEQ
> >       default n
> >       bool "Enabled debugging of rseq() system call" if EXPERT
> > diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> > index 860b2dcf3ac4..04bfb1e4d377 100644
> > --- a/kernel/sys_ni.c
> > +++ b/kernel/sys_ni.c
> > @@ -299,6 +299,7 @@ COND_SYSCALL(set_mempolicy);
> >  COND_SYSCALL(migrate_pages);
> >  COND_SYSCALL(move_pages);
> >  COND_SYSCALL(set_mempolicy_home_node);
> > +COND_SYSCALL(cachestat);
> >
> >  COND_SYSCALL(perf_event_open);
> >  COND_SYSCALL(accept4);
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 08341616ae7a..0305eaf5b3f5 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -22,6 +22,7 @@
> >  #include <linux/mm.h>
> >  #include <linux/swap.h>
> >  #include <linux/swapops.h>
> > +#include <linux/syscalls.h>
> >  #include <linux/mman.h>
> >  #include <linux/pagemap.h>
> >  #include <linux/file.h>
> > @@ -55,6 +56,13 @@
> >  #include <linux/buffer_head.h> /* for try_to_free_buffers */
> >
> >  #include <asm/mman.h>
> > +#include <uapi/linux/mman.h>
> > +
> > +#include "swap.h"
> > +
> > +#ifdef CONFIG_CACHESTAT_SYSCALL
> > +#define LATEST_CACHESTAT_VERSION     1
> > +#endif
> >
> >  /*
> >   * Shared mappings implemented 30.11.1994. It's not fully working yet,
> > @@ -3949,3 +3957,149 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
> >       return try_to_free_buffers(folio);
> >  }
> >  EXPORT_SYMBOL(filemap_release_folio);
> > +
> > +/**
> > + * filemap_cachestat() - compute the page cache statistics of a mapping
> > + * @mapping: The mapping to compute the statistics for.
> > + * @first_index:     The starting page cache index.
> > + * @last_index:      The final page index (inclusive).
> > + * @cs:      the cachestat struct to write the result to.
> > + *
> > + * This will query the page cache statistics of a mapping in the
> > + * page range of [first_index, last_index] (inclusive). The statistics
> > + * queried include: number of dirty pages, number of pages marked for
> > + * writeback, and the number of (recently) evicted pages.
> > + */
> > +void filemap_cachestat(struct address_space *mapping, pgoff_t first_index,
> > +             pgoff_t last_index, struct cachestat *cs)
> > +{
> > +     XA_STATE(xas, &mapping->i_pages, first_index);
> > +     struct folio *folio;
> > +
> > +     rcu_read_lock();
> > +     xas_for_each(&xas, folio, last_index) {
> > +             unsigned long nr_pages;
> > +             pgoff_t folio_first_index, folio_last_index;
> > +
> > +             if (xas_retry(&xas, folio))
> > +                     continue;
> > +
> > +             nr_pages = folio_nr_pages(folio);
> > +             folio_first_index = folio_pgoff(folio);
> > +             folio_last_index = folio_first_index + nr_pages - 1;
> > +
> > +             /* Folios might straddle the range boundaries, only count covered subpages */
> > +             if (folio_first_index < first_index)
> > +                     nr_pages -= first_index - folio_first_index;
> > +
> > +             if (folio_last_index > last_index)
> > +                     nr_pages -= folio_last_index - last_index;
> > +
> > +             if (xa_is_value(folio)) {
> > +                     /* page is evicted */
> > +                     void *shadow = (void *)folio;
> > +                     bool workingset; /* not used */
> > +
> > +                     cs->nr_evicted += nr_pages;
> > +
> > +#ifdef CONFIG_SWAP /* implies CONFIG_MMU */
> > +                     if (shmem_mapping(mapping)) {
> > +                             /* shmem file - in swap cache */
> > +                             swp_entry_t swp = radix_to_swp_entry(folio);
> > +
> > +                             shadow = get_shadow_from_swap_cache(swp);
> > +                     }
> > +#endif
> > +                     if (workingset_test_recent(shadow, true, &workingset))
> > +                             cs->nr_recently_evicted += nr_pages;
> > +
> > +                     goto resched;
> > +             }
> > +
> > +             /* page is in cache */
> > +             cs->nr_cache += nr_pages;
> > +
> > +             if (folio_test_dirty(folio))
> > +                     cs->nr_dirty += nr_pages;
> > +
> > +             if (folio_test_writeback(folio))
> > +                     cs->nr_writeback += nr_pages;
> > +
> > +resched:
> > +             if (need_resched()) {
> > +                     xas_pause(&xas);
> > +                     cond_resched_rcu();
> > +             }
> > +     }
> > +     rcu_read_unlock();
> > +}
> > +EXPORT_SYMBOL(filemap_cachestat);
> > +
> > +#ifdef CONFIG_CACHESTAT_SYSCALL
> > +/*
> > + * The cachestat(5) system call.
> > + *
> > + * cachestat() returns the page cache statistics of a file in the
> > + * bytes range specified by `off` and `len`: number of cached pages,
> > + * number of dirty pages, number of pages marked for writeback,
> > + * number of evicted pages, and number of recently evicted pages.
> > + *
> > + * An evicted page is a page that is previously in the page cache
> > + * but has been evicted since. A page is recently evicted if its last
> > + * eviction was recent enough that its reentry to the cache would
> > + * indicate that it is actively being used by the system, and that
> > + * there is memory pressure on the system.
> > + *
> > + * `off` and `len` must be non-negative integers. If `len` > 0,
> > + * the queried range is [`off`, `off` + `len`]. If `len` == 0,
> > + * we will query in the range from `off` to the end of the file.
> > + *
> > + * `cstat_version` is an unsigned integer indicating the specific version
> > + * of the cachestat struct. It must be at least 1, and does not exceed the
> > + * latest version number (which is currently 1). For now, user should
> > + * just pass 1.
> > + *
> > + * The `flags` argument is unused for now, but is included for future
> > + * extensibility. User should pass 0 (i.e no flag specified).
> > + *
> > + * Because the status of a page can change after cachestat() checks it
> > + * but before it returns to the application, the returned values may
> > + * contain stale information.
> > + *
> > + * return values:
> > + *  zero    - success
> > + *  -EFAULT - cstat points to an illegal address
> > + *  -EINVAL - invalid arguments
> > + *  -EBADF   - invalid file descriptor
> > + */
> > +SYSCALL_DEFINE6(cachestat, unsigned int, fd, off_t, off, size_t, len,
> > +             unsigned int, cstat_version, struct cachestat __user *, cstat,
> > +             unsigned int, flags)
> > +{
> > +     struct fd f = fdget(fd);
> > +     struct address_space *mapping;
> > +     struct cachestat cs;
> > +     pgoff_t first_index = off >> PAGE_SHIFT;
> > +     pgoff_t last_index =
> > +             len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
> > +
> > +     if (!f.file)
> > +             return -EBADF;
> > +
> > +     if (off < 0 || flags != 0 || cstat_version < 1 ||
> > +                     cstat_version > LATEST_CACHESTAT_VERSION) {
> > +             fdput(f);
> > +             return -EINVAL;
> > +     }
> > +
> > +     memset(&cs, 0, sizeof(struct cachestat));
> > +     mapping = f.file->f_mapping;
> > +     filemap_cachestat(mapping, first_index, last_index, &cs);
> > +     fdput(f);
> > +
> > +     if (copy_to_user(cstat, &cs, sizeof(struct cachestat)))
> > +             return -EFAULT;
> > +
> > +     return 0;
> > +}
> > +#endif /* CONFIG_CACHESTAT_SYSCALL */
> > --
> > 2.30.2
> >
>