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Tue, 03 Jun 2025 12:14:54 -0700 (PDT) MIME-Version: 1.0 References: <20250528-hugetlb-nerf-v1-1-a404ca33e819@google.com> <20250602204107.177e2fdf2209b0926b5ce28e@linux-foundation.org> In-Reply-To: From: Jann Horn Date: Tue, 3 Jun 2025 21:14:18 +0200 X-Gm-Features: AX0GCFsFvhvH0fqhCGFfZnff8t58qsDJk6OqTPGqLPaAOhBWr1RNx22ZmLDArvE Message-ID: Subject: Re: [PATCH] hugetlb: block hugetlb file creation if hugetlb is not set up To: Oscar Salvador Cc: Andrew Morton , Muchun Song , linux-mm@kvack.org, Lorenzo Stoakes Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Queue-Id: 3BE681C000F X-Rspamd-Server: rspam09 X-Stat-Signature: sxzib7hxwnwqbwowf79d58bmkis1qnn8 X-HE-Tag: 1748978096-94105 X-HE-Meta: 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 LXhCWkwH 1RiDXKCPSC904E6DsjRVk1jZNK7dJ/0IqD0eJpbGhcuPmsTzDQ2wdsFUOiOg2i6TW5scFEhoo3/3/ONBx6ZLr3EY4q29YejhI77vf+daxS7r2tiD0PmTrgXpLXCQTLjIf0tzmaUy0g3+0DbBMQXIw68UY2HtNVlBqGsiIIU8DB8gSWMTnvuZH807aFnHnwbhU8dfT X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 3, 2025 at 7:43=E2=80=AFAM Oscar Salvador w= rote: > On Tue, Jun 03, 2025 at 06:29:24AM +0200, Jann Horn wrote: > > Yes, this change changes kernel behavior that is userspace-visible, > > and causes syscalls to return errors where they worked before. > > Yes, that is what make me unease about this. > > It is true that most of the hugetlb cases out there work on > pre-allocated pages, because the later it gets the harder to get large > pages from the system. > > But as you say below, there might be applications out there that tweak > the sys knobs themselves, and with this change those might break. > Now, how valid are those? Heh, hard to anwser. > > So I guess it boils down to how hard and effective is to actually exploit > whatever we manage to create by allowing this. > But if we take that route, I think that hinting the user about this behav= iour > change is the right thing to do. I guess the unusual part of hugetlb are really mostly the shared page tables, which are also the reason why walks of hugetlb page tables require extra locks. I guess I'll think about this some more and maybe send a slightly different take on this that only prevents page fault handling from getting far enough to establish shared PMDs, or something like that.