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Wed, 26 Feb 2025 08:34:39 -0800 (PST) Message-ID: <52adb963-9501-4d6b-a2cc-ec0e461baaf0@citrix.com> Date: Wed, 26 Feb 2025 16:34:37 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] xen/page_alloc: Simplify domain_adjust_tot_pages To: Jan Beulich , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: xen-devel@lists.xenproject.org, Anthony PERARD , Michal Orzel , Julien Grall , Stefano Stabellini , Alejandro Vallejo References: <20250224132724.9074-1-alejandro.vallejo@cloud.com> <292f8373-705a-4405-bbdb-af750eb5f0ac@suse.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 26/02/2025 4:06 pm, Jan Beulich wrote: > On 26.02.2025 17:04, Roger Pau Monné wrote: >> On Wed, Feb 26, 2025 at 03:36:33PM +0100, Jan Beulich wrote: >>> On 26.02.2025 15:28, Roger Pau Monné wrote: >>>> On Wed, Feb 26, 2025 at 03:08:33PM +0100, Jan Beulich wrote: >>>>> On 26.02.2025 14:56, Roger Pau Monné wrote: >>>>>> On Mon, Feb 24, 2025 at 01:27:24PM +0000, Alejandro Vallejo wrote: >>>>>>> --- a/xen/common/page_alloc.c >>>>>>> +++ b/xen/common/page_alloc.c >>>>>>> @@ -490,13 +490,11 @@ static long outstanding_claims; /* total outstanding claims by all domains */ >>>>>>> >>>>>>> unsigned long domain_adjust_tot_pages(struct domain *d, long pages) >>>>>>> { >>>>>>> - long dom_before, dom_after, dom_claimed, sys_before, sys_after; >>>>>>> - >>>>>>> ASSERT(rspin_is_locked(&d->page_alloc_lock)); >>>>>>> d->tot_pages += pages; >>>>>>> >>>>>>> /* >>>>>>> - * can test d->claimed_pages race-free because it can only change >>>>>>> + * can test d->outstanding_pages race-free because it can only change >>>>>>> * if d->page_alloc_lock and heap_lock are both held, see also >>>>>>> * domain_set_outstanding_pages below >>>>>>> */ >>>>>>> @@ -504,17 +502,16 @@ unsigned long domain_adjust_tot_pages(struct domain *d, long pages) >>>>>>> goto out; >>>>>> I think you can probably short-circuit the logic below if pages == 0? >>>>>> (and avoid taking the heap_lock) >>>>> Are there callers passing in 0? >>>> Not sure, but if there are no callers expected we might add an ASSERT >>>> to that effect then. >>>> >>>>>>> spin_lock(&heap_lock); >>>>>>> - /* adjust domain outstanding pages; may not go negative */ >>>>>>> - dom_before = d->outstanding_pages; >>>>>>> - dom_after = dom_before - pages; >>>>>>> - BUG_ON(dom_before < 0); >>>>>>> - dom_claimed = dom_after < 0 ? 0 : dom_after; >>>>>>> - d->outstanding_pages = dom_claimed; >>>>>>> - /* flag accounting bug if system outstanding_claims would go negative */ >>>>>>> - sys_before = outstanding_claims; >>>>>>> - sys_after = sys_before - (dom_before - dom_claimed); >>>>>>> - BUG_ON(sys_after < 0); >>>>>>> - outstanding_claims = sys_after; >>>>>>> + BUG_ON(outstanding_claims < d->outstanding_pages); >>>>>>> + if ( pages > 0 && d->outstanding_pages < pages ) >>>>>>> + { >>>>>>> + /* `pages` exceeds the domain's outstanding count. Zero it out. */ >>>>>>> + outstanding_claims -= d->outstanding_pages; >>>>>>> + d->outstanding_pages = 0; >>>>>>> + } else { >>>>>>> + outstanding_claims -= pages; >>>>>>> + d->outstanding_pages -= pages; >>>>>> I wonder if it's intentional for a pages < 0 value to modify >>>>>> outstanding_claims and d->outstanding_pages, I think those values >>>>>> should only be set from domain_set_outstanding_pages(). >>>>>> domain_adjust_tot_pages() should only decrease the value, but never >>>>>> increase either outstanding_claims or d->outstanding_pages. >>>>>> >>>>>> At best the behavior is inconsistent, because once >>>>>> d->outstanding_pages reaches 0 there will be no further modification >>>>>> from domain_adjust_tot_pages(). >>>>> Right, at that point the claim has run out. While freeing pages with an >>>>> active claim means that the claim gets bigger (which naturally needs >>>>> reflecting in the global). >>>> domain_adjust_tot_pages() is not exclusively called when freeing >>>> pages, see steal_page() for example. >>> Or also when pages were allocated. steal_page() ... >>> >>>> When called from steal_page() it's wrong to increase the claim, as >>>> it assumes that the page removed from d->tot_pages is freed, but >>>> that's not the case. The domain might end up in a situation where >>>> the claim is bigger than the available amount of memory. >>> ... is a case that likely wasn't considered when the feature was added. >>> >>> I never really liked this; I'd be quite happy to see it ripped out, as >>> long as we'd be reasonably certain it isn't in active use by people. >> What do you mean with 'it' in the above sentence, the whole claim >> stuff? > Yes. > >> Or just getting rid of allowing the claim to increase as a >> result of pages being removed from a domain? > No. Alejandro and I discussed this earlier in the week. The claim infrastructure stuff is critical for a toolstack capable of doing things in parallel. However, it is also nonsensical for there to be a remaining claim by the time domain construction is done. If creation_finished were a concrete thing, rather than a bodge hacked into domain_unpause_by_systemcontroller(), it ought to be made to fail if there were an outstanding claim.  I suggested that we follow through on a previous suggestion of making it a real hypercall (which is needed by the encrypted VM crowd too). ~Andrew