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Subject: Re: [RFC PATCH 4/8] mm/madvise: define madvise behavior in a struct
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20210927093103.g3cszw75gfctwtzk@box.shutemov.name>
Date:   Mon, 27 Sep 2021 03:31:21 -0700
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan@kernel.org>,
        Colin Cross <ccross@google.com>,
        Suren Baghdasarya <surenb@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>
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Message-Id: <48D4E700-0005-46D4-8EAA-B839D8449C66@gmail.com>
References: <20210926161259.238054-1-namit@vmware.com>
 <20210926161259.238054-5-namit@vmware.com>
 <20210927093103.g3cszw75gfctwtzk@box.shutemov.name>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Precedence: bulk


> On Sep 27, 2021, at 2:31 AM, Kirill A. Shutemov <kirill@shutemov.name> =
wrote:
>=20
> On Sun, Sep 26, 2021 at 09:12:55AM -0700, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>>=20
>> The different behaviors of madvise are different in several ways, =
which
>> are distributed across several functions. Use the design pattern from
>> iouring in order to define the actions that are required for each
>> behavior.
>>=20
>> The next patches will get rid of old helper functions that are =
modified
>> in this patch and the redundant use of array_index_nospec(). The next
>> patches will add more actions for each leaf into the new struct.
>>=20
>> No functional change is intended.
>>=20
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Colin Cross <ccross@google.com>
>> Cc: Suren Baghdasarya <surenb@google.com>
>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>> mm/madvise.c | 168 =
+++++++++++++++++++++++++++++++++------------------
>> 1 file changed, 109 insertions(+), 59 deletions(-)
>>=20
>> diff --git a/mm/madvise.c b/mm/madvise.c
>> index 17e39c70704b..127507c71ba9 100644
>> --- a/mm/madvise.c
>> +++ b/mm/madvise.c
>> @@ -29,6 +29,7 @@
>> #include <linux/swapops.h>
>> #include <linux/shmem_fs.h>
>> #include <linux/mmu_notifier.h>
>> +#include <linux/nospec.h>
>>=20
>> #include <asm/tlb.h>
>>=20
>> @@ -39,6 +40,101 @@ struct madvise_walk_private {
>> 	bool pageout;
>> };
>>=20
>> +struct madvise_info {
>> +	u8 behavior_valid: 1;
>> +	u8 process_behavior_valid: 1;
>> +	u8 need_mmap_read_only: 1;
>> +};
>> +
>> +static const struct madvise_info madvise_info[MADV_SOFT_OFFLINE+1] =3D=
 {
>=20
> MADV_SOFT_OFFLINE+1 smells bad.

I can set another constant instead and let the compiler shout if =
anything
outside the array is initialized.

>=20
> And I don't like the change in general. Given that MADV_SOFT_OFFLINE =
is
> 101, the array will be mostly empty.

Seriously, these is less than 128B - two cachelines. Perhaps they should
be aligned. But this whole change should have no effect on code/data =
size.

>=20
> I donno. I don't see any improvement with the patch. But maybe it's =
only me.

The following patches make it clearer when TLBs flushes are batched and
when mmap_lock is not taken (which is by the way not clear from the =
code).

I could have added two more functions for that and it would have taken
me less time. I do not think the end result of having ~5 different
functions to figure out the actions needed for each behavior would be
as clear.=