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mm: properly fork shared file mappings

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mm_fork_context() currently maps shared chdev mappings 1:1 when forking
a range. The issue with this approach is that chdev mappings are not
guaranteed to be backed by physical memory, and thus cannot safely be
refcounted. This can lead to issues on devices with more complex buffer
lifetimes.

To allow for internal book-keeping, do not map shared chdev memory on
fork and instead allow it to go through the entire fault-in process
again.
This commit is contained in:
Matheus
2026-10-06 10:47:05 -03:00
parent 0cdd03eba8
commit 6c65a90a5f
1 changed files with 17 additions and 9 deletions
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kernel-src/mm/mm.c
+17 -9
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@@ -590,21 +590,27 @@ mm_context_t *mm_fork_context(mm_context_t *old_context) {
bool writeable = arch_mmu_iswritable(old_context->pagetable, vaddr);
bool private = !(range->flags & MM_RANGE_FLAGS_SHARED);
if ((range->flags & MM_RANGE_FLAGS_FILE) && range->vnode->type == V_TYPE_CHDEV && !private) {
// TODO: implement fork semantics for device mappings properly.
if (arch_mmu_map(new_context->pagetable, phys, vaddr, new_range->mmuflags) == false)
goto error;
// the semantics of device mappings in general are:
// - a device mapping always gets *fully* mapped on VOP_MMAP
// - if a device mapping is private, it gets a valid
// physical page and goes through the normal fork path.
// - if a device mapping is shared, it does not have a valid
// physical page and must have VOP_MMAP called for it again.
// defer that until its actually accessed in a new page fault
if ((range->flags & MM_RANGE_FLAGS_FILE) &&
range->vnode->type == V_TYPE_CHDEV && !private)
continue;
}
page_t *page = mm_get_page(phys);
if (mm_is_page_locked(page) && writeable && private) {
// the page is a private locked page, eagerly copy it to ensure fork/futex correctness
// the page is a private locked page, eagerly copy it
// to ensure fork/futex correctness
void *new_phys = mm_alloc_page(MEMORY_SECTION_DEFAULT);
if (new_phys == NULL)
goto error;
if (arch_mmu_map(new_context->pagetable, new_phys, vaddr, new_range->mmuflags) == false) {
if (arch_mmu_map(new_context->pagetable, new_phys, vaddr,
new_range->mmuflags) == false) {
mm_release_page(new_phys);
goto error;
}
@@ -612,12 +618,14 @@ mm_context_t *mm_fork_context(mm_context_t *old_context) {
memcpy(MAKE_HHDM(new_phys), MAKE_HHDM(phys), PAGE_SIZE);
} else {
// go through the normal CoW path
if (arch_mmu_map(new_context->pagetable, phys, vaddr, new_range->mmuflags & ~ARCH_MMU_FLAGS_WRITE) == false)
if (arch_mmu_map(new_context->pagetable, phys, vaddr,
new_range->mmuflags & ~ARCH_MMU_FLAGS_WRITE) == false)
goto error;
mm_hold_page(phys);
arch_mmu_remap(old_context->pagetable, phys, vaddr, new_range->mmuflags & ~ARCH_MMU_FLAGS_WRITE);
arch_mmu_remap(old_context->pagetable, phys, vaddr,
new_range->mmuflags & ~ARCH_MMU_FLAGS_WRITE);
}
}