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U2FsdGVkX1+lhlQuZNhGRcnSbbstQfwjyDxmmPT0OJK6YTSqA8NBsMJZTCbiSPMb+Jj341nQYX0AkizAP1e78yqk9k4bHC0lz9LJ/hiTSyj1w8GX//4YAXyzKHRu34pprjcNIoM3HAExK760JsEPUKKOJvbPKlMkJpHRm5ZU0NvSBVZbsV2u7BDQAX2KGz8BXkmVZKZC74OMhJEyCpiFymgrWcgPK9HZcZV7tZUx3C/WQ4A21acMN1mGE6SfBj1Neqad+RYnPGanriDbbu3LCx5ayg7Kq7Vl5BIoqndIrrOFMetVFe6U10yjsEmw8UkmvT7q/JF/XB7J/NSXeRe1AO5R+K7dWi+pOckMGNR18pPCxv5D72S43s9t4VI+2eAZKrtOQPwHiC4PjicdNu87ThpX22XPUiY63C7BWZj5v/Za4KTDbLiE+QbM/HdZIEvkazRkciVJRU+0Fh6HLomWpewwQeja82PIgLXVsgx1NDD0SUP3rXU71nSCDqOTgYoRwAudpVO7RkhWsEkuKIKRyHe3GZ0MYeQHarxzPSQTKalelsPBs/TQtQtdDlRpE9JqhuzLYd1UOvObIP6j572glAORW65eyqQoDYP3rCOSOIZShKs+jrhHtfSIbSfHSD69orUQySrOACK8+knFIO38XpqvgtsqoLlJyuYLt9cirtGzMt0DD5G6KhsRAzzR6sxf5D1xtZEMCB1dddl5RhifbFnadC1u05LTtE/UyU5NVC3n3M1nRzwtZFL2ypOknIRPx0TI3cxNITjh9YdpzQYa8xtDVCPP6CWixjkx0a4wAlzXIho+WCl9C2fbAC4x/n+BSDTTjQKmbff/lWtr9zIGtxtABsGUpW3o80NBoN5J1ecWBZ+qB9vY/ZUoPAw/iwvCUR2vMkzDqGL0T7ogu8lXxx4q9EyCJdqYSMWz6Aq1KXIDEOEVEOZNh8y2+mbKifSIjAGqqIwnNT2jAl4uzBO Ul+BvSGz nA/FT5DopGZqq3RESmjl4EZYIp8+x4FjCJCneXmuEZj8ZHrQww8gS/H76ShliFixtlmJFCcvZWyp49yHQz0ITfb/ubA== 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, May 20, 2025 at 06:26:09PM -0400, Johannes Weiner wrote: > On Mon, May 19, 2025 at 09:52:41PM +0100, Lorenzo Stoakes wrote: > > We intentionally limit this only to flags that we know should function > > correctly without issue, and to be conservative about this, so we initially > > limit ourselves only to MADV_HUGEPAGE, MADV_NOHUGEPAGE, that is - setting > > the VM_HUGEPAGE, VM_NOHUGEPAGE VMA flags. > > Hm, but do we actually expect more to show up? Looking at the current > list of advisories, we have the conventional ones: > > MADV_NORMAL > No special treatment. This is the default. > > MADV_RANDOM > Expect page references in random order. (Hence, read ahead may be less useful than normally.) > > MADV_SEQUENTIAL > Expect page references in sequential order. (Hence, pages in the given range can be aggressively read ahead, and may be freed soon after they are accessed.) > > MADV_WILLNEED > Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.) > > MADV_DONTNEED > Do not expect access in the near future. (For the time being, the application is finished with the given range, so the kernel can free resources associated with it.) > > where only RANDOM and SEQUENTIAL are actual policies. But since those > apply to file mappings only, they don't seem to make much sense for a > process-wide setting. > > For Linux-specific advisories we have > > MADV_REMOVE - action > MADV_DONTFORK - policy, but not sure how this could work as a process-wide thing > MADV_HWPOISON - action > MADV_MERGEABLE - policy, but we have a prctl() for process-wide settings > MADV_SOFTOFFLINE - action > MADV_HUGEPAGE - policy, but we have a prctl() for process-wide disabling > MADV_COLLAPSE - action > MADV_DONTDUMP - policy, but there is /proc//coredump_filter for process-wide settings > MADV_FREE - action > MADV_WIPEONFORK - policy, but similar to DONTFORK, not sure how this would work process-wide > MADV_COLD - action > MADV_PAGEOUT - action > MADV_POPULATE_READ - action > MADV_POPULATE_WRITE - action > MADV_GUARD_INSTALL - action > > So the vast majority of advisories are either one-off actions, or they > are policies that naturally only make sense for very specific ranges. > > KSM and THP really seem like the notable exception[1], rather than a > rule. And we already *have* prctl() ops to modify per-process policies > for these. So I'm reluctant to agree we should drill open and expand > madvise() to cover them - especially with the goal or the expectation > that THP per-process policies shouldn't matter that much down the line. > > I will admit, I don't hate prctl() as much as you do. It *is* a fairly > broad interface - setting per-process policies. So it's bound to pick > odds and ends of various subsystems that don't quite fit elsewhere. > > Is it the right choice everywhere? Of course not. And can its > broadness be abused to avoid real interface design? Absolutely. > > I don't think that makes it inherently bad, though. As long as we make > an honest effort to find the best home for new knobs. > > But IMO, in this case it is. The inheritance-along-the-process-tree > behavior that we want here is an established pattern in prctl(). > > And *because* that propagation is a security-sensitive pattern > (although I don't think the THP policy specifically *is* a security > issue), to me it makes more sense to keep this behavior confined to > prctl() at least, and not add it to madvise too. > > [1] Maybe we should have added sys_andrea() to cover those, as well as > the SECCOMP prctl()s ;) So sorry to have missed you really excellent and well thought-out reply here Johannes (grr mail etc.). You make a really good point, and I think this does overall, sadly, suggest the concept of a 'general' madvise() call while, in a sense, 'neat', doesn't quite fit. I do think it aligns well with a dedicated mctl() API call, have sent out an RFC discussion thread about this so we can determine if this makes sense. Thanks, Lorenzo